Khronos Vulkan

Layer Interface to the Loader

Creative Commons

Table of Contents

Overview

This is the Layer-centric view of working with the Vulkan loader. For the complete overview of all sections of the loader, please refer to the LoaderInterfaceArchitecture.html file.

Layer Discovery

As mentioned in the Implicit versus Explicit, section of the LoaderApplicationInterface.html document, layers can be categorized into two categories:

The main difference between the two is that implicit layers are automatically enabled, unless overridden, and explicit layers must be enabled. Remember, implicit layers are not present on all Operating Systems (like Android).

On any system, the loader looks in specific areas for information on the layers that it can load at a user's request. The process of finding the available layers on a system is known as Layer Discovery. During discovery, the loader determines what layers are available, the layer name, the layer version, and any extensions supported by the layer. This information is provided back to an application through vkEnumerateInstanceLayerProperties.

The group of layers available to the loader is known as the Layer Library. This section defines an extensible interface to discover what layers are contained in the Layer Library.

This section also specifies the minimal conventions and rules a layer must follow, especially with regards to interacting with the loader and other layers.

When searching for a layer, the loader will look through the Layer Library in the order it detected them and load the layer if the name matches. If multiple instances of the same library exist in different locations throughout the user's system, then the one appearing first in the search order will be used. Each OS has its own search order that is defined in its layer discovery section below. If multiple manifest files in the same directory define the same layer, but point to different library files, the order which the layers is loaded is random due to the behavior of readdir.

Additionally, any duplicate layer names in either the component layer list, or globally among all enabled layers, during calls to vkCreateInstance or vkCreateDevice will simply be ignored by the loader. Only the first occurrence of any layer name will be used.

Layer Manifest File Usage

On Windows, Linux, and macOS systems, JSON-formatted manifest files are used to store layer information. In order to find system-installed layers, the Vulkan loader will read the JSON files to identify the names and attributes of layers and their extensions. The use of manifest files allows the loader to avoid loading any shared library files when the application does not query nor request any extensions. The format of Layer Manifest File is detailed below.

The Android loader does not use manifest files. Instead, the loader queries the layer properties using special functions known as "introspection" functions. The intent of these functions is to determine the same required information gathered from reading the manifest files. These introspection functions are not used by the Khronos loader but should be present in layers to maintain consistency. The specific "introspection" functions are called out in the Layer Manifest File Format table.

Android Layer Discovery

On Android, the loader looks for layers to enumerate in the /data/local/debug/vulkan folder. An application enabled for debug has the ability to enumerate and enable any layers in that location.

Windows Layer Discovery

In order to find system-installed layers, the Vulkan loader will scan the values in the following Windows registry keys:

HKEY_LOCAL_MACHINE\SOFTWARE\Khronos\Vulkan\ExplicitLayers
HKEY_CURRENT_USER\SOFTWARE\Khronos\Vulkan\ExplicitLayers
HKEY_LOCAL_MACHINE\SOFTWARE\Khronos\Vulkan\ImplicitLayers
HKEY_CURRENT_USER\SOFTWARE\Khronos\Vulkan\ImplicitLayers

Except when running a 32-bit application on 64-bit Windows, when the loader will instead scan the 32-bit registry location:

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Khronos\Vulkan\ExplicitLayers
HKEY_CURRENT_USER\SOFTWARE\WOW6432Node\Khronos\Vulkan\ExplicitLayers
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Khronos\Vulkan\ImplicitLayers
HKEY_CURRENT_USER\SOFTWARE\WOW6432Node\Khronos\Vulkan\ImplicitLayers

For each value in these keys which has DWORD data set to 0, the loader opens the JSON manifest file specified by the name of the value. Each name must be an absolute path to the manifest file. Additionally, the HKEY_CURRENT_USER locations will only be searched if an application is not being executed with administrative privileges. This is done to ensure that an application with administrative privileges does not run layers that did not need administrator access to install.

Because some layers are installed alongside drivers, the loader will scan through registry keys specific to Display Adapters and all Software Components associated with these adapters for the locations of JSON manifest files. These keys are located in device keys created during driver installation and contain configuration information for base settings, including Vulkan, OpenGL, and Direct3D ICD location.

The Device Adapter and Software Component key paths should be obtained through the PnP Configuration Manager API. The 000X key will be a numbered key, where each device is assigned a different number.

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Class\{Adapter GUID}\000X\VulkanExplicitLayers
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Class\{Adapter GUID}\000X\VulkanImplicitLayers
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Class\{Software Component GUID}\000X\VulkanExplicitLayers
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Class\{Software Component GUID}\000X\VulkanImplicitLayers

In addition, on 64-bit systems there may be another set of registry values, listed below. These values record the locations of 32-bit layers on 64-bit operating systems, in the same way as the Windows-on-Windows functionality.

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Class\{Adapter GUID}\000X\VulkanExplicitLayersWow
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Class\{Adapter GUID}\000X\VulkanImplicitLayersWow
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Class\{Software Component GUID}\000X\VulkanExplicitLayersWow
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Class\{Software Component GUID}\000X\VulkanImplicitLayersWow

If any of the above values exist and is of type REG_SZ, the loader will open the JSON manifest file specified by the key value. Each value must be an absolute path to a JSON manifest file. A key value may also be of type REG_MULTI_SZ, in which case the value will be interpreted as a list of paths to JSON manifest files.

In general, applications should install layers into the SOFTWARE\Khronos\Vulkan paths. The PnP registry locations are intended specifically for layers that are distributed as part of a driver installation. An application installer should not modify the device-specific registries, while a device driver should not modify the system registries.

The Vulkan loader will open each manifest file to obtain information about the layer, including the name or pathname of a shared library (".dll") file.

If VK_LAYER_PATH is defined, then the loader will look at the paths defined by that variable for explicit layer manifest files instead of using the information provided by the explicit layer registry keys.

If VK_ADD_LAYER_PATH is defined, then the loader will look at the provided paths for explicit layer manifest files in addition to using the information provided by the explicit layer registry keys. The paths provided by VK_ADD_LAYER_PATH are added before the standard list of search folders and will therefore be searched first.

If VK_LAYER_PATH is present, then VK_ADD_LAYER_PATH will not be used by the loader and any values will be ignored.

For security reasons, both VK_LAYER_PATH and VK_ADD_LAYER_PATH are ignored if running with elevated privileges. See Exception for Elevated Privileges for more info.

See Forcing Layer Source Folders in the LoaderApplicationInterface.html document for more information on this.

Linux Layer Discovery

On Linux, the Vulkan loader will scan for manifest files using environment variables or corresponding fallback values if the corresponding environment variable is not defined:

Search Order Directory/Environment Variable Fallback Additional Notes
1 $XDG_CONFIG_HOME $HOME/.config This path is ignored when running with elevated privileges such as setuid, setgid, or filesystem capabilities.
This is done because under these scenarios it is not safe to trust that the environment variables are non-malicious.
1 $XDG_CONFIG_DIRS /etc/xdg
2 SYSCONFDIR /etc Compile-time option set to possible location of layers installed from non-Linux-distribution-provided packages.
3 EXTRASYSCONFDIR /etc Compile-time option set to possible location of layers installed from non-Linux-distribution-provided packages. Typically only set if SYSCONFDIR is set to something other than /etc
4 $XDG_DATA_HOME $HOME/.local/share This path is ignored when running with elevated privileges such as setuid, setgid, or filesystem capabilities.
This is done because under these scenarios it is not safe to trust that the environment variables are non-malicious.
5 $XDG_DATA_DIRS /usr/local/share/:/usr/share/

The directory lists are concatenated together using the standard platform path separator (:). The loader then selects each path, and applies a suffix onto it for the specific type of layer being searched for and looks in that specific folder for manifest files:

If VK_LAYER_PATH is defined, then the loader will look at the paths defined by that variable for explicit layer manifest files instead of using the information provided by the standard explicit layer paths mentioned above.

If VK_ADD_LAYER_PATH is defined, then the loader will look at the provided paths for explicit layer manifest files in addition to using the information provided by the standard explicit layer paths mentioned above. The paths provided by VK_ADD_LAYER_PATH are added before the standard list of search folders and will therefore be searched first.

If VK_LAYER_PATH is present, then VK_ADD_LAYER_PATH will not be used by the loader and any values will be ignored.

For security reasons, both VK_LAYER_PATH and VK_ADD_LAYER_PATH are ignored if running with elevated privileges. See Exception for Elevated Privileges for more info.

NOTE While the order of folders searched for manifest files is well defined, the order contents are read by the loader in each directory is random due to the behavior of readdir.

See Forcing Layer Source Folders in the LoaderApplicationInterface.html document for more information on this.

It is also important to note that while both VK_LAYER_PATH and VK_ADD_LAYER_PATH will point the loader paths to search for finding the manifest files, it does not guarantee the library files mentioned by the manifest will immediately be found. Often, the layer manifest file will point to the library file using a relative or absolute path. When a relative or absolute path is used, the loader can typically find the library file without querying the operating system. However, if a library is listed only by name, the loader may not find it. If problems occur finding a library file associated with a layer, try updating the LD_LIBRARY_PATH environment variable to point at the location of the corresponding .so file.

Example Linux Explicit Layer Search Path

For a fictional user "me" the layer manifest search path might look like the following:

  /home/me/.config/vulkan/explicit_layer.d
  /etc/xdg/vulkan/explicit_layer.d
  /usr/local/etc/vulkan/explicit_layer.d
  /etc/vulkan/explicit_layer.d
  /home/me/.local/share/vulkan/explicit_layer.d
  /usr/local/share/vulkan/explicit_layer.d
  /usr/share/vulkan/explicit_layer.d

Fuchsia Layer Discovery

On Fuchsia, the Vulkan loader will scan for manifest files using environment variables or corresponding fallback values if the corresponding environment variable is not defined in the same way as Linux. The only difference is that Fuchsia does not allow fallback values for $XDG_DATA_DIRS or $XDG_HOME_DIRS.

macOS Layer Discovery

On macOS, the Vulkan loader will scan for manifest files using the application resource folder as well as environment variables or corresponding fallback values if the corresponding environment variable is not defined. The order is similar to the search path on Linux with the exception that the application's bundle resources are searched first: (bundle)/Contents/Resources/.

Example macOS Implicit Layer Search Path

For a fictional user "Me" the layer manifest search path might look like the following:

  <bundle>/Contents/Resources/vulkan/implicit_layer.d
  /Users/Me/.config/vulkan/implicit_layer.d
  /etc/xdg/vulkan/implicit_layer.d
  /usr/local/etc/vulkan/implicit_layer.d
  /etc/vulkan/implicit_layer.d
  /Users/Me/.local/share/vulkan/implicit_layer.d
  /usr/local/share/vulkan/implicit_layer.d
  /usr/share/vulkan/implicit_layer.d

Layer Filtering

NOTE: This functionality is only available with Loaders built with version 1.3.234 of the Vulkan headers and later.

The loader supports filter environment variables which can forcibly enable and disable known layers. Known layers are those that are already found by the loader taking into account default search paths and other environment variables (like VK_LAYER_PATH or VK_ADD_LAYER_PATH).

The filter variables will be compared against the layer name provided in the layer's manifest file.

The filters must also follow the behaviors define in the Filter Environment Variable Behaviors section of the LoaderLayerInterface document.

Layer Enable Filtering

The layer enable environment variable VK_LOADER_LAYERS_ENABLE is a comma-delimited list of globs to search for in known layers. The layer names are compared against the globs listed in the environment variable, and if they match, they will automatically be added to the enabled layer list in the loader for each application. These layers are enabled after implicit layers but before other explicit layers.

When a layer is enabled using the VK_LOADER_LAYERS_ENABLE filter, and loader logging is set to emit either warnings or layer messages, then a message will show for each layer that has been forced on. This message will look like the following:

WARNING | LAYER:  Layer "VK_LAYER_LUNARG_wrap_objects" force enabled due to env var 'VK_LOADER_LAYERS_ENABLE'

Layer Disable Filtering

The layer disable environment variable VK_LOADER_LAYERS_DISABLE is a comma-delimited list of globs to search for in known layers. The layer names are compared against the globs listed in the environment variable, and if they match, they will automatically be disabled (whether or not the layer is Implicit or Explicit). This means that they will not be added to the enabled layer list in the loader for each application. This could mean that layers requested by an application are also not enabled such as VK_KHRONOS_LAYER_synchronization2 which could cause some applications to misbehave.

When a layer is disabled using the VK_LOADER_LAYERS_DISABLE filter, and loader logging is set to emit either warnings or layer messages, then a message will show for each layer that has been forcibly disabled. This message will look like the following:

WARNING | LAYER:  Layer "VK_LAYER_LUNARG_wrap_objects" disabled because name matches filter of env var 'VK_LOADER_LAYERS_DISABLE'

Layer Special Case Disable

Because there are different types of layers, there are 3 additional special disable options available when using the VK_LOADER_LAYERS_DISABLE environment variable.

These are:

~all~ will effectively disable every layer. This enables a developer to disable all layers on the system. ~implicit~ will effectively disable every implicit layer (leaving explicit layers still present in the application call chain). ~explicit~ will effectively disable every explicit layer (leaving implicit layers still present in the application call chain).

Layer Disable Warning

Disabling layers, whether just through normal usage of VK_LOADER_LAYERS_DISABLE or by evoking one of the special disable options like ~all~ or ~explicit~ could cause application breakage if the application is relying on features provided by one or more explicit layers.

VK_INSTANCE_LAYERS

The original VK_INSTANCE_LAYERS can be viewed as a special case of the new VK_LOADER_LAYERS_ENABLE. Because of this, any layers enabled via VK_INSTANCE_LAYERS will be treated the same as layers enabled with VK_LOADER_LAYERS_ENABLE and will therefore override any disables supplied in VK_LOADER_LAYERS_DISABLE.

Exception for Elevated Privileges

For security reasons, VK_LAYER_PATH and VK_ADD_LAYER_PATH are ignored if running the Vulkan application with elevated privileges. This is because they may insert new libraries into the executable process that are not normally found by the loader. Because of this, these environment variables can only be used for applications that do not use elevated privileges.

For more information see Elevated Privilege Caveats in the top-level [LoaderInterfaceArchitecture.html][LoaderInterfaceArchitecture.html] document.

Layer Version Negotiation

Now that a layer has been discovered, an application can choose to load it, or in the case of implicit layers, it can be loaded by default. When the loader attempts to load the layer, the first thing it does is attempt to negotiate the version of the loader to layer interface. In order to negotiate the loader/layer interface version, the layer must implement the vkNegotiateLoaderLayerInterfaceVersion function. The following information is provided for this interface in include/vulkan/vk_layer.h:

typedef enum VkNegotiateLayerStructType {
    LAYER_NEGOTIATE_INTERFACE_STRUCT = 1,
} VkNegotiateLayerStructType;

typedef struct VkNegotiateLayerInterface {
    VkNegotiateLayerStructType sType;
    void *pNext;
    uint32_t loaderLayerInterfaceVersion;
    PFN_vkGetInstanceProcAddr pfnGetInstanceProcAddr;
    PFN_vkGetDeviceProcAddr pfnGetDeviceProcAddr;
    PFN_GetPhysicalDeviceProcAddr pfnGetPhysicalDeviceProcAddr;
} VkNegotiateLayerInterface;

VkResult
   vkNegotiateLoaderLayerInterfaceVersion(
      VkNegotiateLayerInterface *pVersionStruct);

The VkNegotiateLayerInterface structure is similar to other Vulkan structures. The "sType" field, in this case takes a new enum defined just for internal loader/layer interfacing use. The valid values for "sType" could grow in the future, but right now only has the one value "LAYER_NEGOTIATE_INTERFACE_STRUCT".

This function (vkNegotiateLoaderLayerInterfaceVersion) should be exported by the layer so that using "GetProcAddress" on Windows or "dlsym" on Linux or macOS, should return a valid function pointer to it. Once the loader has grabbed a valid address to the layers function, the loader will create a variable of type VkNegotiateLayerInterface and initialize it in the following ways:

  1. Set the structure "sType" to "LAYER_NEGOTIATE_INTERFACE_STRUCT"
  2. Set pNext to NULL.
    • This is for future growth
  3. Set "loaderLayerInterfaceVersion" to the current version the loader desires to set the interface to.
    • The minimum value sent by the loader will be 2 since it is the first version supporting this function.

The loader will then individually call each layer’s vkNegotiateLoaderLayerInterfaceVersion function with the filled out “VkNegotiateLayerInterface”.

This function allows the loader and layer to agree on an interface version to use. The "loaderLayerInterfaceVersion" field is both an input and output parameter. "loaderLayerInterfaceVersion" is filled in by the loader with the desired latest interface version supported by the loader (typically the latest). The layer receives this and returns back the version it desires in the same field. Because it is setting up the interface version between the loader and layer, this should be the first call made by a loader to the layer (even prior to any calls to vkGetInstanceProcAddr).

If the layer receiving the call no longer supports the interface version provided by the loader (due to deprecation), then it should report a VK_ERROR_INITIALIZATION_FAILED error. Otherwise it sets the value pointed by "loaderLayerInterfaceVersion" to the latest interface version supported by both the layer and the loader and returns VK_SUCCESS.

The layer should report VK_SUCCESS in case the loader-provided interface version is newer than that supported by the layer, as it's the loader's responsibility to determine whether it can support the older interface version supported by the layer. The layer should also report VK_SUCCESS in the case its interface version is greater than the loader's, but return the loader's version. Thus, upon return of VK_SUCCESS the "loaderLayerInterfaceVersion" will contain the desired interface version to be used by the layer.

If the loader receives VK_ERROR_INITIALIZATION_FAILED instead of VK_SUCCESS, then the loader will treat the layer as unusable and will not load it. In this case, the application will not see the layer during enumeration. Note that the loader is currently backwards compatible with all layer interface versions, so a layer should not be able to request a version older than what the loader supports.

This function MUST NOT call down the layer chain to the next layer. The loader will work with each layer individually.

If the layer supports the new interface and reports version 2 or greater, then The layer should fill in the function pointer values to its internal functions: - "pfnGetInstanceProcAddr" should be set to the layer’s internal GetInstanceProcAddr function. - "pfnGetDeviceProcAddr" should be set to the layer’s internal GetDeviceProcAddr function. - "pfnGetPhysicalDeviceProcAddr" should be set to the layer’s internal GetPhysicalDeviceProcAddr function. - If the layer supports no physical device extensions, it may set the value to NULL. - More on this function later the loader will use the “fpGetInstanceProcAddr” and “fpGetDeviceProcAddr” functions from the “VkNegotiateLayerInterface” structure. Prior to these changes, the loader would query each of those functions using "GetProcAddress" on Windows or "dlsym" on Linux or macOS.

Layer Call Chains and Distributed Dispatch

There are two key architectural features that drive the loader to Layer Library interface:

  1. Separate and distinct instance and device call chains
  2. Distributed dispatch.

For further information, read the overview of dispatch tables and call chains above in the Dispatch Tables and Call Chains section of the LoaderInterfaceArchitecture.html document.

What's important to note here is that a layer can intercept Vulkan instance functions, device functions or both. For a layer to intercept instance functions, it must participate in the instance call chain. For a layer to intercept device functions, it must participate in the device call chain.

Remember, a layer does not need to intercept all instance or device functions, instead, it can choose to intercept only a subset of those functions.

Normally, when a layer intercepts a given Vulkan function, it will call down the instance or device call chain as needed. The loader and all layer libraries that participate in a call chain cooperate to ensure the correct sequencing of calls from one entity to the next. This group effort for call chain sequencing is hereinafter referred to as distributed dispatch.

In distributed dispatch each layer is responsible for properly calling the next entity in the call chain. This means that a dispatch mechanism is required for all Vulkan functions that a layer intercepts. If a Vulkan function is not intercepted by a layer, or if a layer chooses to terminate the function by not calling down the chain, then no dispatch is needed for that particular function.

For example, if the enabled layers intercepted only certain instance functions, the call chain would look as follows: Instance Function Chain

Likewise, if the enabled layers intercepted only a few of the device functions, the call chain could look this way: Device Function Chain

The loader is responsible for dispatching all core and instance extension Vulkan functions to the first entity in the call chain.

Layer Unknown Physical Device Extensions

Layers that intercept entrypoints which take a VkPhysicalDevice as the first parameter should support vk_layerGetPhysicalDeviceProcAddr. This function is added to the Layer Interface Version 2 and allows the loader to distinguish between entrypoints which take VkDevice and VkPhysicalDevice as the first parameter. This allows the loader to properly support entrypoints that are unknown to it gracefully.

PFN_vkVoidFunction
   vk_layerGetPhysicalDeviceProcAddr(
      VkInstance instance,
      const char* pName);

This function behaves similar to vkGetInstanceProcAddr and vkGetDeviceProcAddr except it should only return values for physical device extension entry-points. In this way, it compares "pName" to every physical device function supported in the layer.

Implementations of the function should have the following behavior:

If a layer intends to support functions that take VkPhysicalDevice as the dispatchable parameter, then layer should support vk_layerGetPhysicalDeviceProcAddr. This is because if these functions aren't known to the loader, such as those from unreleased extensions or because the loader is an older build thus doesn't know about them yet, the loader won't be able to distinguish whether this is a device or physical device function.

If a layer does implement vk_layerGetPhysicalDeviceProcAddr, it should return the address of its vk_layerGetPhysicalDeviceProcAddr function in the "pfnGetPhysicalDeviceProcAddr" member of the VkNegotiateLayerInterface structure during Layer Version Negotiation. Additionally, the layer should also make sure vkGetInstanceProcAddr returns a valid function pointer to a query of vk_layerGetPhysicalDeviceProcAddr.

Note: If a layer wraps the VkInstance handle, support for vk_layerGetPhysicalDeviceProcAddr is NOT optional and must be implemented.

The behavior of the loader's vkGetInstanceProcAddr with support for the vk_layerGetPhysicalDeviceProcAddr function is as follows:

  1. Check if core function:
    • If it is, return the function pointer
  2. Check if known instance or device extension function:
    • If it is, return the function pointer
  3. Call the layer/driver GetPhysicalDeviceProcAddr
    • If it returns non-NULL, return a trampoline to a generic physical device function, and set up a generic terminator which will pass it to the proper driver.
  4. Call down using GetInstanceProcAddr
    • If it returns non-NULL, treat it as an unknown logical device command. This means setting up a generic trampoline function that takes in a VkDevice as the first parameter and adjusting the dispatch table to call the driver/layer's function after getting the dispatch table from the VkDevice. Then, return the pointer to corresponding trampoline function.
  5. Return NULL

Then, if the command gets promoted to core later, it will no longer be set up using vk_layerGetPhysicalDeviceProcAddr. Additionally, if the loader adds direct support for the extension, it will no longer get to step 3, because step 2 will return a valid function pointer. However, the layer should continue to support the command query via vk_layerGetPhysicalDeviceProcAddr, until at least a Vulkan version bump, because an older loader may still be attempting to use the commands.

Reason for adding vk_layerGetPhysicalDeviceProcAddr

Originally, if vkGetInstanceProcAddr was called in the loader, it would result in the following behavior:

  1. The loader would check if core function:
    • If it was, it would return the function pointer
  2. The loader would check if known extension function:
    • If it was, it would return the function pointer
  3. If the loader knew nothing about it, it would call down using GetInstanceProcAddr
    • If it returned non-NULL, treat it as an unknown logical device command.
    • This meant setting up a generic trampoline function that takes in a VkDevice as the first parameter and adjusting the dispatch table to call the Driver/Layer's function after getting the dispatch table from the VkDevice.
  4. If all the above failed, the loader would return NULL to the application.

This caused problems when a layer attempted to expose new physical device extensions the loader knew nothing about, but an application did. Because the loader knew nothing about it, the loader would get to step 3 in the above process and would treat the function as an unknown logical device command. The problem is, this would create a generic VkDevice trampoline function which, on the first call, would attempt to dereference the VkPhysicalDevice as a VkDevice. This would lead to a crash or corruption.

Layer Intercept Requirements

Distributed Dispatching Requirements

Layer Conventions and Rules

A layer, when inserted into an otherwise compliant Vulkan driver, must still result in a compliant Vulkan driver. The intention is for layers to have a well-defined baseline behavior. Therefore, it must follow some conventions and rules defined below.

In order for layers to have unique names, and reduce the chance of conflicts that could occur when the loader attempts to load these layers, layers must adhere to the following naming standard:

Examples of valid layer names include:

More details on layer naming can be found in the Vulkan style-guide under section 3.4 "Version, Extension, and Layer Naming Conventions".

A layer is always chained with other layers. It must not make invalid calls to, or rely on undefined behaviors of, its lower layers. When it changes the behavior of a function, it must make sure its upper layers do not make invalid calls to or rely on undefined behaviors of its lower layers because of the changed behavior. For example, when a layer intercepts an object creation function to wrap the objects created by its lower layers, it must make sure its lower layers never see the wrapping objects, directly from itself or indirectly from its upper layers.

When a layer requires host memory, it may ignore the provided allocators. It is preferred that the layer use any provided memory allocators if the layer is intended to run in a production environment. For example, this usually applies to implicit layers that are always enabled. That will allow applications to include the layer's memory usage.

Additional rules include:

Layer Dispatch Initialization

Example Code for CreateInstance

VkResult
   vkCreateInstance(
      const VkInstanceCreateInfo *pCreateInfo,
      const VkAllocationCallbacks *pAllocator,
      VkInstance *pInstance)
{
   VkLayerInstanceCreateInfo *chain_info =
        get_chain_info(pCreateInfo, VK_LAYER_LINK_INFO);

    assert(chain_info->u.pLayerInfo);
    PFN_vkGetInstanceProcAddr fpGetInstanceProcAddr =
        chain_info->u.pLayerInfo->pfnNextGetInstanceProcAddr;
    PFN_vkCreateInstance fpCreateInstance =
        (PFN_vkCreateInstance)fpGetInstanceProcAddr(NULL, "vkCreateInstance");
    if (fpCreateInstance == NULL) {
        return VK_ERROR_INITIALIZATION_FAILED;
    }

    // Advance the link info for the next element of the chain.
    // This ensures that the next layer gets it's layer info and not
    // the info for our current layer.
    chain_info->u.pLayerInfo = chain_info->u.pLayerInfo->pNext;

    // Continue call down the chain
    VkResult result = fpCreateInstance(pCreateInfo, pAllocator, pInstance);
    if (result != VK_SUCCESS)
        return result;

    // Init layer's dispatch table using GetInstanceProcAddr of
    // next layer in the chain.
    instance_dispatch_table = new VkLayerInstanceDispatchTable;
    layer_init_instance_dispatch_table(
        *pInstance, my_data->instance_dispatch_table, fpGetInstanceProcAddr);

    // Other layer initialization
    ...

    return VK_SUCCESS;
}

Example Code for CreateDevice

VkResult
   vkCreateDevice(
      VkPhysicalDevice gpu,
      const VkDeviceCreateInfo *pCreateInfo,
      const VkAllocationCallbacks *pAllocator,
      VkDevice *pDevice)
{
    VkInstance instance = GetInstanceFromPhysicalDevice(gpu);
    VkLayerDeviceCreateInfo *chain_info =
        get_chain_info(pCreateInfo, VK_LAYER_LINK_INFO);

    PFN_vkGetInstanceProcAddr fpGetInstanceProcAddr =
        chain_info->u.pLayerInfo->pfnNextGetInstanceProcAddr;
    PFN_vkGetDeviceProcAddr fpGetDeviceProcAddr =
        chain_info->u.pLayerInfo->pfnNextGetDeviceProcAddr;
    PFN_vkCreateDevice fpCreateDevice =
        (PFN_vkCreateDevice)fpGetInstanceProcAddr(instance, "vkCreateDevice");
    if (fpCreateDevice == NULL) {
        return VK_ERROR_INITIALIZATION_FAILED;
    }

    // Advance the link info for the next element on the chain.
    // This ensures that the next layer gets it's layer info and not
    // the info for our current layer.
    chain_info->u.pLayerInfo = chain_info->u.pLayerInfo->pNext;

    VkResult result = fpCreateDevice(gpu, pCreateInfo, pAllocator, pDevice);
    if (result != VK_SUCCESS) {
        return result;
    }

    // initialize layer's dispatch table
    device_dispatch_table = new VkLayerDispatchTable;
    layer_init_device_dispatch_table(
        *pDevice, device_dispatch_table, fpGetDeviceProcAddr);

    // Other layer initialization
    ...

    return VK_SUCCESS;
}

In this case the function GetInstanceFromPhysicalDevice is called to get the instance handle. In practice, this would be done by any method a layer chooses to get an instance handle from the physical device.

Meta-layers

Meta-layers are a special kind of layer which is only available through the Khronos loader. While normal layers are associated with one particular library, a meta-layer is actually a collection layer which contains an ordered list of other layers (called component layers).

The benefits of a meta-layer are:

  1. More than one layer may be activated using a single layer name by simply grouping multiple layers in a meta-layer.
  2. The order of individual component layers is loaded can be defined within the meta-layer.
  3. Layer configurations (internal to the meta-layer manifest file) can easily be shared with others.
  4. The loader will automatically collate all instance and device extensions in a meta-layer's component layers, and report them as the meta-layer's properties to the application when queried.

Restrictions to defining and using a meta-layer are:

  1. A Meta-layer Manifest file must be a properly formatted that contains one or more component layers.
  2. All component layers must be present on a system for the meta-layer to be used.
  3. All component layers must be at the same Vulkan API major and minor version as the meta-layer for the meta-layer to be used.

The ordering of a meta-layer's component layers in the instance or device call- chain is simple:

Inside the meta-layer Manifest file, each component layer is listed by its layer name. This is the "name" tag's value associated with each component layer's Manifest file under the "layer" or "layers" tag. This is also the name that would normally be used when activating a layer during vkCreateInstance.

Any duplicate layer names in either the component layer list, or globally among all enabled layers, will simply be ignored by the loader. Only the first instance of any layer name will be used.

For example, if a layer is enabled using the environment variable VK_INSTANCE_LAYERS and have that same layer listed in a meta-layer, then the environment-variable-enabled layer will be used and the component layer will be dropped. Likewise, if a person were to enable a meta-layer and then separately enable one of the component layers afterwards, the second instantiation of the layer name would be ignored.

The Manifest file formatting necessary to define a meta-layer can be found in the Layer Manifest File Format section.

Override Meta-Layer

If an implicit meta-layer was found on the system with the name VK_LAYER_LUNARG_override, the loader uses it as an 'override' layer. This is used to selectively enable and disable other layers from being loaded. It can be applied globally or to a specific application or applications. The override meta layer can have the following additional keys:

When an application starts up and the override layer is present, the loader first checks to see if the application is in the list. If it isn't, the override layer is not applied. If the list is empty or if app_keys doesn't exist, the loader makes the override layer global and applies it to every application upon startup.

If the override layer contains override_paths, then it uses this list of paths exclusively for component layers. Thus, it ignores both the default explicit and implicit layer layer search locations as well as paths set by environment variables like VK_LAYER_PATH. If any component layer is not present in the provided override paths, the meta layer is disabled.

The override meta-layer is primarily enabled when using the VkConfig tool included in the Vulkan SDK. It is typically only available while the VkConfig tool is actually executing. Please refer to that documentation for more information.

Pre-Instance Functions

Vulkan includes a small number of functions which are called without any dispatchable object. Most layers do not intercept these functions, as layers are enabled when an instance is created. However, under certain conditions it is possible for a layer to intercept these functions.

One reason why a layer may desire to intercept these pre-instance functions is to filter out extensions that would normally be returned from Vulkan drivers to the application. RenderDoc is one such layer which intercepts these pre-instance functions so that it may disable extensions it doesn't support.

In order to intercept the pre-instance functions, several conditions must be met:

The functions that may be intercepted in this way are:

Pre-instance functions work differently from all other layer intercept functions. Other intercept functions have a function prototype identical to that of the function they are intercepting. They then rely on data that was passed to the layer at instance or device creation so that layers can call down the chain. Because there is no need to create an instance before calling the pre-instance functions, these functions must use a separate mechanism for constructing the call chain. This mechanism consists of an extra parameter that will be passed to the layer intercept function when it is called. This parameter will be a pointer to a struct, defined as follows:

typedef struct Vk...Chain
{
    struct {
        VkChainType type;
        uint32_t version;
        uint32_t size;
    } header;
    PFN_vkVoidFunction pfnNextLayer;
    const struct Vk...Chain* pNextLink;
} Vk...Chain;

These structs are defined in the vk_layer.h file so that it is not necessary to redefine the chain structs in any external code. The name of each struct is be similar to the name of the function it corresponds to, but the leading "V" is capitalized, and the word "Chain" is added to the end. For example, the struct for vkEnumerateInstanceExtensionProperties is called VkEnumerateInstanceExtensionPropertiesChain. Furthermore, the pfnNextLayer struct member is not actually a void function pointer — its type will be the actual type of each function in the call chain.

Each layer intercept function must have a prototype that is the same as the prototype of the function being intercepted, except that the first parameter must be that function's chain struct (passed as a const pointer). For example, a function that wishes to intercept vkEnumerateInstanceExtensionProperties would have the prototype:

VkResult
   InterceptFunctionName(
      const VkEnumerateInstanceExtensionPropertiesChain* pChain,
      const char* pLayerName,
      uint32_t* pPropertyCount,
      VkExtensionProperties* pProperties);

The name of the function is arbitrary; it can be anything provided that it is given in the layer manifest file (see Layer Manifest File Format). The implementation of each intercept function is responsible for calling the next item in the call chain, using the chain parameter. This is done by calling the pfnNextLayer member of the chain struct, passing pNextLink as the first argument, and passing the remaining function arguments after that. For example, a simple implementation for vkEnumerateInstanceExtensionProperties that does nothing but call down the chain would look like:

VkResult
   InterceptFunctionName(
      const VkEnumerateInstanceExtensionPropertiesChain* pChain,
      const char* pLayerName,
      uint32_t* pPropertyCount,
      VkExtensionProperties* pProperties)
{
   return pChain->pfnNextLayer(
      pChain->pNextLink, pLayerName, pPropertyCount, pProperties);
}

When using a C++ compiler, each chain type also defines a function named CallDown which can be used to automatically handle the first argument. Implementing the above function using this method would look like:

VkResult
   InterceptFunctionName(
      const VkEnumerateInstanceExtensionPropertiesChain* pChain,
      const char* pLayerName,
      uint32_t* pPropertyCount,
      VkExtensionProperties* pProperties)
{
   return pChain->CallDown(pLayerName, pPropertyCount, pProperties);
}

Unlike with other functions in layers, the layer may not save any global data between these function calls. Because Vulkan does not store any state until an instance has been created, all layer libraries are released at the end of each pre-instance call. This means that implicit layers can use pre-instance intercepts to modify data that is returned by the functions, but they cannot be used to record that data.

Special Considerations

Associating Private Data with Vulkan Objects Within a Layer

A layer may want to associate its own private data with one or more Vulkan objects. Two common methods to do this are hash maps and object wrapping.

Wrapping

The loader supports layers wrapping any Vulkan object, including dispatchable objects. For functions that return object handles, each layer does not touch the value passed down the call chain. This is because lower items may need to use the original value. However, when the value is returned from a lower-level layer (possibly the driver), the layer saves the handle and returns its own handle to the layer above it (possibly the application). When a layer receives a Vulkan function using something that it previously returned a handle for, the layer is required to unwrap the handle and pass along the saved handle to the layer below it. This means that the layer must intercept every Vulkan function which uses the object in question, and wrap or unwrap the object, as appropriate. This includes adding support for all extensions with functions using any object the layer wraps as well as any loader-layer interface functions such as vk_layerGetPhysicalDeviceProcAddr.

Layers above the object wrapping layer will see the wrapped object. Layers which wrap dispatchable objects must ensure that the first field in the wrapping structure is a pointer to a dispatch table as defined in vk_layer.h. Specifically, an instance wrapped dispatchable object could be as follows:

struct my_wrapped_instance_obj_ {
    VkLayerInstanceDispatchTable *disp;
    // whatever data layer wants to add to this object
};

A device wrapped dispatchable object could be as follows:

struct my_wrapped_instance_obj_ {
    VkLayerDispatchTable *disp;
    // whatever data layer wants to add to this object
};

Layers that wrap dispatchable objects must follow the guidelines for creating new dispatchable objects (below).

Cautions About Wrapping

Layers are generally discouraged from wrapping objects, because of the potential for incompatibilities with new extensions. For example, let's say that a layer wraps VkImage objects, and properly wraps and unwraps VkImage object handles for all core functions. If a new extension is created which has functions that take VkImage objects as parameters, and if the layer does not support those new functions, an application that uses both the layer and the new extension will have undefined behavior when those new functions are called (e.g. the application may crash). This is because the lower-level layers and drivers won't receive the handle that they generated. Instead, they will receive a handle that is only known by the layer that is wrapping the object.

Because of the potential for incompatibilities with unsupported extensions, layers that wrap objects must check which extensions are being used by the application, and take appropriate action if the layer is used with unsupported extensions such as issuing a warning/error message to the user.

The reason that the validation layers wrap objects is to track the proper use and destruction of each object. They issue a validation error if used with unsupported extensions, alerting the user to the potential for undefined behavior.

Hash Maps

Alternatively, a layer may want to use a hash map to associate data with a given object. The key to the map could be the object. Alternatively, for dispatchable objects at a given level (eg device or instance) the layer may want data associated with the VkDevice or VkInstance objects. Since there are multiple dispatchable objects for a given VkInstance or VkDevice, the VkDevice or VkInstance object is not a great map key. Instead the layer should use the dispatch table pointer within the VkDevice or VkInstance since that will be unique for a given VkInstance or VkDevice.

Creating New Dispatchable Objects

Layers which create dispatchable objects must take special care. Remember that loader trampoline code normally fills in the dispatch table pointer in the newly created object. Thus, the layer must fill in the dispatch table pointer if the loader trampoline will not do so. Common cases where a layer (or driver) may create a dispatchable object without loader trampoline code is as follows:

The Khronos loader provides a callback that can be used for initializing a dispatchable object. The callback is passed as an extension structure via the pNext field in the create info structure when creating an instance (VkInstanceCreateInfo) or device (VkDeviceCreateInfo). The callback prototype is defined as follows for instance and device callbacks respectively (see vk_layer.h):

VKAPI_ATTR VkResult VKAPI_CALL
   vkSetInstanceLoaderData(
      VkInstance instance,
      void *object);

VKAPI_ATTR VkResult VKAPI_CALL
   vkSetDeviceLoaderData(
      VkDevice device,
      void *object);

To obtain these callbacks the layer must search through the list of structures pointed to by the "pNext" field in the VkInstanceCreateInfo and VkDeviceCreateInfo parameters to find any callback structures inserted by the loader. The salient details are as follows:

Alternatively, if an older loader is being used that doesn't provide these callbacks, the layer may manually initialize the newly created dispatchable object. To fill in the dispatch table pointer in newly created dispatchable object, the layer should copy the dispatch pointer, which is always the first entry in the structure, from an existing parent object of the same level (instance versus device).

For example, if there is a newly created VkCommandBuffer object, then the dispatch pointer from the VkDevice object, which is the parent of the VkCommandBuffer object, should be copied into the newly created object.

Versioning and Activation Interactions

There are several interacting rules concerning the activation of layers with non-obvious results. This not an exhaustive list but should better clarify the behavior of the loader in complex situations.

Layer Manifest File Format

The Khronos loader uses manifest files to discover available layer libraries and layers. It doesn't directly query the layer's dynamic library except during chaining. This is to reduce the likelihood of loading a malicious layer into memory. Instead, details are read from the Manifest file, which are then provided for applications to determine what layers should actually be loaded.

The following section discusses the details of the Layer Manifest JSON file format. The JSON file itself does not have any requirements for naming. The only requirement is that the extension suffix of the file is ".json".

Here is an example layer JSON Manifest file with a single layer:

{
   "file_format_version" : "1.2.1",
   "layer": {
       "name": "VK_LAYER_LUNARG_overlay",
       "type": "INSTANCE",
       "library_path": "vkOverlayLayer.dll",
       "library_arch" : "64",
       "api_version" : "1.0.5",
       "implementation_version" : "2",
       "description" : "LunarG HUD layer",
       "functions": {
           "vkNegotiateLoaderLayerInterfaceVersion":
               "OverlayLayer_NegotiateLoaderLayerInterfaceVersion"
       },
       "instance_extensions": [
           {
               "name": "VK_EXT_debug_report",
               "spec_version": "1"
           },
           {
               "name": "VK_VENDOR_ext_x",
               "spec_version": "3"
            }
       ],
       "device_extensions": [
           {
               "name": "VK_EXT_debug_marker",
               "spec_version": "1",
               "entrypoints": ["vkCmdDbgMarkerBegin", "vkCmdDbgMarkerEnd"]
           }
       ],
       "enable_environment": {
           "ENABLE_LAYER_OVERLAY_1": "1"
       },
       "disable_environment": {
           "DISABLE_LAYER_OVERLAY_1": ""
       }
   }
}

Here's a snippet with the changes required to support multiple layers per manifest file:

{
   "file_format_version" : "1.0.1",
   "layers": [
      {
           "name": "VK_LAYER_layer_name1",
           "type": "INSTANCE",
           ...
      },
      {
           "name": "VK_LAYER_layer_name2",
           "type": "INSTANCE",
           ...
      }
   ]
}

Here's an example of a meta-layer manifest file:

{
   "file_format_version" : "1.1.1",
   "layer": {
       "name": "VK_LAYER_META_layer",
       "type": "GLOBAL",
       "api_version" : "1.0.40",
       "implementation_version" : "1",
       "description" : "LunarG Meta-layer example",
       "component_layers": [
           "VK_LAYER_KHRONOS_validation",
           "VK_LAYER_LUNARG_api_dump"
       ]
   }
}
JSON Node Description and Notes Restrictions Parent Introspection Query
"api_version" The major.minor.patch version number of the Vulkan API that the layer supports. It does not require the application to make use of that API version. It simply is an indication that the layer can support Vulkan API instance and device functions up to and including that API version.
For example: 1.0.33. 		None "layer"/"layers" vkEnumerateInstanceLayerProperties
"app_keys" List of paths to executables that the meta-layer applies to. Meta-layers Only "layer"/"layers" N/A
"blacklisted_layers" List of explicit layer names that should not be loaded even if requested by the application. Meta-layers Only "layer"/"layers" N/A
"component_layers" Indicates the component layer names that are part of a meta-layer. The names listed must be the "name" identified in each of the component layer's Mainfest file "name" tag (this is the same as the name of the layer that is passed to the `vkCreateInstance` command). All component layers must be present on the system and found by the loader in order for this meta-layer to be available and activated.
This field must not be present if "library_path" is defined. 		Meta-layers Only "layer"/"layers" N/A
"description" A high-level description of the layer and its intended use. None "layer"/"layers" vkEnumerateInstanceLayerProperties
"device_extensions" OPTIONAL: Contains the list of device extension names supported by this layer. One "device\_extensions" node with an array of one or more elements is required if any device extensions are supported by a layer; otherwise the node is optional. Each element of the array must have the nodes "name" and "spec_version" which correspond to `VkExtensionProperties` "extensionName" and "specVersion" respectively. Additionally, each element of the array of device extensions must have the node "entrypoints" if the device extension adds Vulkan API functions; otherwise this node is not required. The "entrypoint" node is an array of the names of all entry-points added by the supported extension. None "layer"/"layers" vkEnumerateDeviceExtensionProperties
"disable_environment" REQUIRED: Indicates an environment variable used to disable the Implicit Layer (when defined to any non-empty string value).
In rare cases of an application not working with an implicit layer, the application can set this environment variable (before calling Vulkan functions) in order to "blacklist" the layer. This environment variable (which may vary with each variation of the layer) must be set (not particularly to any value). If both the "enable_environment" and "disable_environment" variables are set, the implicit layer is disabled. 		Implicit Layers Only "layer"/"layers" N/A
"enable_environment" OPTIONAL: Indicates an environment variable used to enable the Implicit Layer (when defined to any non-empty string value).
This environment variable (which may vary with each variation of the layer) must be set to the given value or else the implicit layer is not loaded. This is for application environments (e.g. Steam) which want to enable a layer(s) only for applications that they launch, and allows for applications run outside of an application environment to not get that implicit layer(s). 		Implicit Layers Only "layer"/"layers" N/A
"file_format_version" Manifest format major.minor.patch version number.
Supported versions are: 1.0.0, 1.0.1, 1.1.0, 1.1.1, 1.1.2 and 1.2.0. 		None None N/A
"functions" OPTIONAL: This section can be used to identify a different function name for the loader to use in place of standard layer interface functions. The "functions" node is required if the layer is using an alternative name for `vkNegotiateLoaderLayerInterfaceVersion`. None "layer"/"layers" vkGet*ProcAddr
"implementation_version" The version of the layer implemented. If the layer itself has any major changes, this number should change so the loader and/or application can identify it properly. None "layer"/"layers" vkEnumerateInstanceLayerProperties
"instance_extensions" OPTIONAL: Contains the list of instance extension names supported by this layer. One "instance_extensions" node with an array of one or more elements is required if any instance extensions are supported by a layer; otherwise the node is optional. Each element of the array must have the nodes "name" and "spec_version" which correspond to `VkExtensionProperties` "extensionName" and "specVersion" respectively. None "layer"/"layers" vkEnumerateInstanceExtensionProperties
"layer" The identifier used to group a single layer's information together. None None vkEnumerateInstanceLayerProperties
"layers" The identifier used to group multiple layers' information together. This requires a minimum Manifest file format version of 1.0.1. None None vkEnumerateInstanceLayerProperties
"library_path" Specifies either a filename, a relative pathname, or a full pathname to a layer shared library file. If "library_path" specifies a relative pathname, it is relative to the path of the JSON manifest file (e.g. for cases when an application provides a layer that is in the same folder hierarchy as the rest of the application files). If "library_path" specifies a filename, the library must live in the system's shared object search path. There are no rules about the name of the layer shared library files other than it should end with the appropriate suffix (".DLL" on Windows, ".so" on Linux, and ".dylib" on macOS).
This field must not be present if "component_layers" is defined. 		Not Valid For Meta-layers "layer"/"layers" N/A
"library_arch" Optional field which specifies the architecture of the binary associated with "library_path".
Allows the loader to quickly determine if the architecture of the layer matches that of the running application.
The only valid values are "32" and "64".		 N/A
"name" The string used to uniquely identify this layer to applications. None "layer"/"layers" vkEnumerateInstanceLayerProperties
"override_paths" List of paths which will be used as the search location for component layers. Meta-layers Only "layer"/"layers" N/A
"pre_instance_functions" OPTIONAL: Indicates which functions the layer wishes to intercept, that do not require that an instance has been created. This should be an object where each function to be intercepted is defined as a string entry where the key is the Vulkan function name and the value is the name of the intercept function in the layer's dynamic library. Available in layer manifest versions 1.1.2 and up.
See Pre-Instance Functions for more information. 		Implicit Layers Only "layer"/"layers" vkEnumerateInstance*Properties
"type" This field indicates the type of layer. The values can be: GLOBAL, or INSTANCE.
NOTE: Prior to deprecation, the "type" node was used to indicate which layer chain(s) to activate the layer upon: instance, device, or both. Distinct instance and device layers are deprecated; there are now just instance layers. Originally, allowable values were "INSTANCE", "GLOBAL" and, "DEVICE." But now "DEVICE" layers are skipped over by the loader as if they were not found. 		None "layer"/"layers" vkEnumerate*LayerProperties

Layer Manifest File Version History

The current highest supported Layer Manifest file format supported is 1.2.0. Information about each version is detailed in the following sub-sections:

Layer Manifest File Version 1.2.1

Added the "library_arch" field to the layer manifest to allow the loader to quickly determine if the layer matches the architecture of the current running application.

Layer Manifest File Version 1.2.0

The ability to define the layer settings as defined by the layer manifest schema.

The ability to briefly document the layer thanks to the fields:

These changes were made to enable third-party layers to expose their features within Vulkan Configurator or other tools.

Layer Manifest File Version 1.1.2

Version 1.1.2 introduced the ability of layers to intercept function calls that do not have an instance.

Layer Manifest File Version 1.1.1

The ability to define custom metalayers was added. To support metalayers, the "component_layers" section was added, and the requirement for a "library_path" section to be present was removed when the "component_layers" section is present.

Layer Manifest File Version 1.1.0

Layer Manifest File Version 1.1.0 is tied to changes exposed by the Loader/Layer interface version 2.

  1. Renaming "vkGetInstanceProcAddr" in the "functions" section is deprecated since the loader no longer needs to query the layer about "vkGetInstanceProcAddr" directly. It is now returned during the layer negotiation, so this field will be ignored.
  2. Renaming "vkGetDeviceProcAddr" in the "functions" section is deprecated since the loader no longer needs to query the layer about "vkGetDeviceProcAddr" directly. It too is now returned during the layer negotiation, so this field will be ignored.
  3. Renaming the "vkNegotiateLoaderLayerInterfaceVersion" function is being added to the "functions" section, since this is now the only function the loader needs to query using OS-specific calls.
    • NOTE: This is an optional field and, as the two previous fields, only needed if the layer requires changing the name of the function for some reason.

The layer manifest file does not need to to be updated if the names of any listed functions has not changed.

Layer Manifest File Version 1.0.1

The ability to define multiple layers using the "layers" array was added. This JSON array field can be used when defining a single layer or multiple layers. The "layer" field is still present and valid for a single layer definition.

Layer Manifest File Version 1.0.0

The initial version of the layer manifest file specified the basic format and fields of a layer JSON file. The fields of the 1.0.0 file format include:

It was also during this time that the value of "DEVICE" was deprecated from the "type" field.

Layer Interface Versions

The current loader/layer interface is at version 2. The following sections detail the differences between the various versions.

Layer Interface Version 2

Introduced the concept of loader and layer interface using the vkNegotiateLoaderLayerInterfaceVersion function. Additionally, it introduced the concept of Layer Unknown Physical Device Extensions and the associated vk_layerGetPhysicalDeviceProcAddr function. Finally, it changed the manifest file definition to 1.1.0.

Note: If a layer wraps the VkInstance handle, support for vk_layerGetPhysicalDeviceProcAddr is NOT optional and must be implemented.

Layer Interface Version 1

A layer supporting interface version 1 had the following behavior:

  1. GetInstanceProcAddr and GetDeviceProcAddr were directly exported
  2. The layer manifest file was able to override the names of the GetInstanceProcAddr and GetDeviceProcAddrfunctions.

Layer Interface Version 0

A layer supporting interface version 0 must define and export these introspection functions, unrelated to any Vulkan function despite the names, signatures, and other similarities:

It must also define and export these functions once for each layer in the library:

All layers contained within a library must support vk_layer.h. They do not need to implement functions that they do not intercept. They are recommended not to export any functions.

Loader and Layer Interface Policy

This section is intended to define proper behavior expected between the loader and layers. Much of this section is additive to the Vulkan spec, and necessary for maintaining consistency across platforms. In fact, much of the language can be found throughout this document, but is summarized here for convenience. Additionally, there should be a way to identify bad or non-conformant behavior in a layer and remedy it as soon as possible. Therefore, a policy numbering system is provided to clearly identify each policy statement in a unique way.

Finally, based on the goal of making the loader efficient and performant, some of these policy statements defining proper layer behavior may not be testable (and therefore aren't enforceable by the loader). However, that should not detract from the requirement in order to provide the best experience to end-users and developers.

Number Format

Loader/Layer policy items start with the prefix LLP_ (short for Loader/Layer Policy) which is followed by an identifier based on what component the policy is targeted against. In this case there are only two possible components:

Android Differences

As stated before, the Android Loader is actually separate from the Khronos Loader. Because of this and other platform requirements, not all of these policy statements apply to Android. Each table also has a column titled "Applicable to Android?" which indicates which policy statements apply to layers that are focused only on Android support. Further information on the Android loader can be found in the Android Vulkan documentation.

Requirements of Well-Behaved Layers

Requirement Number Requirement Description Result of Non-Compliance Applicable to Android? Enforceable by Loader? Reference Section
LLP_LAYER_1 A layer, when inserted into an otherwise compliant Vulkan environment, must still result in a compliant Vulkan environment unless it intends to mimic non-compliant behavior (such as a device simulation layer). The behavior is undefined and may result in crashes or corruption. Yes No
It is not a simple task for the loader to find the cause of failure in a layer chain.		 Layer Conventions and Rules
LLP_LAYER_2 A layer must not cause other layers or drivers to fail, crash, or otherwise misbehave.
It must not make invalid calls to, or rely on undefined behaviors of the layers or drivers below it. 		The behavior is undefined and may result in crashes or corruption. Yes No
It is not a simple task for the loader to find the cause of failure in a layer chain.		 Layer Conventions and Rules
LLP_LAYER_3 Any new layer developed should adhere to the naming rules defined in the "Layer Conventions and Rules" section which also correspond to the naming rules defined in the Vulkan Style Guide section 3.4 on "Version, Extension, and Layer Naming Conventions". Layer developers could produce conflicting names causing unexpected behavior if more than one layer with the same name is available on a user's platform. Yes Yes
Can not immediately enforce since it will cause some shipping layers to stop working.		 Vulkan Style Guide section 3.4
Layer Conventions and Rules
LLP_LAYER_4 A layer should export the vkNegotiateLoaderLayerInterfaceVersion entry-point to negotiate an interface version.
A layer using interface 2 or newer must export this function.
 The layer will not be loaded. No Yes Layer Version Negotiation
LLP_LAYER_5 A layer must be able to negotiate a supported version of the loader/layer interface with the loader in accordance with the stated negotiation process. The layer will not be loaded. No Yes Interface Negotiation
LLP_LAYER_6 A layer must have a valid JSON manifest file for the loader to process that ends with the ".json" suffix. It is recommended validating the layer manifest file against the layer schema prior to publication.
The only exception is on Android which determines layer functionality through the introspection functions defined in Layer Library API Version 0 section and in the Layer Manifest File Format table. 		The layer will not be loaded. No Yes Manifest File Usage
LLP_LAYER_7 If a layer is a Meta-layer, each component layer in its manifest file must be present on the system. The layer will not be loaded. No Yes Meta-Layers
LLP_LAYER_8 If a layer is a Meta-layer, each component layer in its manifest file must report the same or a newer Vulkan API major and minor version than the meta-layer. The layer will not be loaded. No Yes Meta-Layers
LLP_LAYER_9 A layer installed as an Implicit Layer must define a disable environment variable so it can be disabled globally. The layer will not be loaded if it does not define the environment variable. Yes Yes Manifest File Format, see "disable_environment" variable
LLP_LAYER_10 If a layer wraps individual object handles, it must unwrap those handles when passing the handles down the chain to the next layer. The behavior is undefined and may result in crashes or corruption. Yes No Cautions About Wrapping
LLP_LAYER_11 Any layer shipped with a driver must be validated against conformance with the corresponding driver. The behavior is undefined and may result in crashes or corruption. Yes No Vulkan CTS Documentation
LLP_LAYER_12 During vkCreateInstance, a layer must process the VkLayerInstanceCreateInfo chain links appropriately.
This includes getting the next layer's vkGetInstanceProcAddr function for building a dispatch table as well as updating the VkLayerInstanceCreateInfo chain link to point to the next structure in the chain for the next layer prior to calling down to the next layer's vkCreateInstance function.
An example of such usage is shown in detail in the Example Code for CreateInstance section. 		The behavior will result in crashes or corruption as any following layers will access incorrect content. Yes No
With the current loader/layer design, it is difficult for the loader to diagnose this without adding additional overhead that could impact performance.
This is because the loader calls all layers at once and has no data on the intermediate state of the pNext chain contents. This could be done in the future, but requires re-designing the layer initialization process. 		Layer Dispatch Initialization
LLP_LAYER_13 During vkCreateDevice, a layer must process the VkLayerDeviceCreateInfo chain links appropriately.
This includes updating the VkLayerDeviceCreateInfo chain link to point to the next structure in the chain for the next layer prior to calling down to the next layer's vkCreateDevice function.
An example of such usage is shown in detail in the Example Code for CreateDevice section. 		The behavior will result in crashes or corruption as any following layers will access incorrect content. Yes No
With the current loader/layer design, it is difficult for the loader to diagnose this without adding additional overhead that could impact performance.		 Layer Dispatch Initialization
LLP_LAYER_14 A layer should use the application provided memory allocator functions when they are provided so that applications can keep track of allocated memory. The allocator functions may be provided for the purpose of limiting or tracking the memory used by the Vulkan components. Because of this, if a layer ignores these allocators, it may result in undefined behavior possibly including crashes or corruption. Yes No Layer Conventions and Rules
LLP_LAYER_15 A layer must enumerate only its own extension properties during a call of vkEnumerateInstanceExtensionProperties when pLayerName refers to itself.
Otherwise, it must return VK_ERROR_LAYER_NOT_PRESENT, including when pLayerName is NULL. 		The loader could become confused on what support is present in a specific layer which will result in undefined behavior possibly including crashes or corruption. Yes No Layer Conventions and Rules
LLP_LAYER_16 A layer must enumerate only its own extension properties during a call of vkEnumerateDeviceExtensionProperties when pLayerName refers to itself.
Otherwise, it must ignore the call other than passing it down the standard call chain. 		The loader could become confused on what support is present in a specific layer which will result in undefined behavior possibly including crashes or corruption. Yes No Layer Conventions and Rules
LLP_LAYER_17 A layer's vkCreateInstance must not generate an error for unrecognized extension names as the extension could be implemented by a lower layer or driver. The behavior is undefined and may result in crashes or corruption. Yes Yes Layer Conventions and Rules
LLP_LAYER_18 A layer must return NULL from vkGetInstanceProcAddr or vkGetDeviceProcAddr for entry-points that it does not support or that have not been enabled properly (for example not enabling the extension certain entry-points are associated with should result in vkGetInstanceProcAddr returning NULL when requesting them). The behavior is undefined and may result in crashes or corruption. Yes No
With the current loader/layer design, it is difficult for the loader to determine this without adding additional overhead that could impact performance.		 Layer Conventions and Rules
LLP_LAYER_19 If a layer creates dispatchable objects, either because it is wrapping objects or implementing an extension not supported by the loader or underlying drivers, it must create the dispatch table appropriately for all created dispatchable objects. The behavior is undefined and may result in crashes or corruption. Yes No Creating New Dispatchable Objects
LLP_LAYER_20 A layer must remove all manifest files and references to those files (i.e. Registry entries on Windows) when uninstalling.
Similarly, on updating the layer files, the old files must be all updated or removed. 		The loader ignores duplicate attempts to load the same manifest file, but if an old file is left pointing to an incorrect library, it will result in undefined behavior which may include crashes or corruption. No No
The loader has no idea what layer files are new, old, or incorrect. Any type of layer file verification would quickly become very complex since it would require the loader to maintain an internal database tracking badly behaving layers based on the layer name, version, targeted platform(s), and possibly other criteria. 		N/A
LLP_LAYER_21 During vkCreateInstance, a layer must not modify the pInstance pointer during prior to calling down to the lower layers.
This is because the loader passes information in this pointer that is necessary for the initialization code in the loader's terminator function.
Instead, if the layer is overriding the pInstance pointer, it must do so only after the call to the lower layers returns. 		The loader will likely crash. No Yes N/A

Requirements of a Well-Behaved Loader

Requirement Number Requirement Description Result of Non-Compliance Applicable to Android? Reference Section
LLP_LOADER_1 A loader must support Vulkan layers. Users will not have access to critical parts of the Vulkan ecosystem such as Validation Layers, GfxReconstruct, or RenderDoc. Yes N/A
LLP_LOADER_2 A loader must support a mechanism to load layers in one or more non-standard locations.
This is to allow application/engine-specific layers as well as evaluating in-development layers without global installation. 		It will be more difficult to use a Vulkan loader by certain tools and driver developers. No Layer Discovery
LLP_LOADER_3 A loader must filter out duplicate layer names in the various enable lists, keeping only the first occurrence. The behavior is undefined and may result in crashes or corruption. Yes Layer Discovery
LLP_LOADER_4 A loader must not load a Vulkan layer which defines an API version that is incompatible with itself. The behavior is undefined and may result in crashes or corruption. Yes Layer Discovery
LLP_LOADER_5 A loader must ignore any layer for which a compatible interface version can not be negotiated. The loader would load a layer improperly resulting in undefined behavior which may include crashes or corruption. No Interface Negotiation
LLP_LOADER_6 If a layer is implicit, and it has an enable environment variable, then a loader must not consider the layer enabled unless that enable environment variable is defined.
If an implicit layer does not have an enable environment variable, it will be considered enabled by default. 		Some layers may be used when not intended. No Manifest File Format, see "enable_environment" variable
LLP_LOADER_7 If an implicit layer is enabled, but has been disabled by some other mechanism (such as the defining of the layer's disable environment variable or through the blacklisting mechanism of the Override Layer), then a loader must not load that layer. Some layers may be used when not intended. No Manifest File Format, see "disable_environment" variable
LLP_LOADER_8 A loader must pass a linked list of initialization structures to each layer via the VkLayerInstanceCreateInfo structure in the pNext field of the VkInstanceCreateInfo structure. This contains necessary information for setting up the instance call chain including providing a function pointer to the next links vkGetInstanceProcAddr. Layers will crash as they attempt to load invalid data. Yes Layer Dispatch Initialization
LLP_LOADER_9 A loader must pass a linked list of initialization structures to each layer via the VkLayerDeviceCreateInfo structure in the pNext field of the VkDeviceCreateInfo structure. This contains necessary information for setting up the device call chain including providing a function pointer to the next links vkGetDeviceProcAddr. Layers will crash as they attempt to load invalid data. Yes Layer Dispatch Initialization
LLP_LOADER_10 A loader must verify that all meta-layers contain valid component layers that the loader can find on the system and that also report the same Vulkan API version as the meta-layer itself before it loads the meta-layer. The behavior is undefined and may result in crashes or corruption. No Meta-Layers
LLP_LOADER_11 If the override meta-layer is present, a loader must load it and corresponding component layers after all other implicit layers have been added to the call chain. The behavior is undefined and may result in crashes or corruption. No Override Meta-Layer
LLP_LOADER_12 If the override meta-layer is present and has a blacklist of layers to remove, a loader must disable all layers listed in the blacklist. The behavior is undefined and may result in crashes or corruption. No Override Meta-Layer
LLP_LOADER_13 A loader must not load from user-defined paths (including the use of either VK_LAYER_PATH or VK_ADD_LAYER_PATH environment variables) when running elevated (Administrator/Super-user) applications.
This is for security reasons. 		The behavior is undefined and may result in computer security lapses, crashes or corruption. No Layer Discovery

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