All layer libraries must support the layer library interface defined in
Layer libraries can be written to intercept or hook VK entry points for various debug and validation purposes. One or more VK entry points can be defined in your Layer library. Undefined entrypoints in the Layer library will be passed to the next Layer which may be the driver. Multiple layer libraries can be chained (actually a hierarchy) together. vkEnumerateInstanceLayerProperties can be called to list the available layers and their properties. Layers can intercept all Vulkan commands that take a dispatchable object as it's first argument. I.e. VkInstance, VkPhysicalDevice, VkDevice, VkCommandBuffer, and VkQueue. vkXXXXGetProcAddr is used internally by the Layers and Loader to initialize dispatch tables. Layers can also be activated via the VK_INSTANCE_LAYERS environment variable.
All validation layers work with the DEBUG_REPORT extension to provide validation feedback. When a validation layer is enabled, it will look for a vk_layer_settings.txt file (see“Using Layers” section below for more details) to define its logging behavior, which can include sending output to a file, stdout, or debug output (Windows). Applications can also register debug callback functions via the DEBUG_REPORT extension to receive callbacks when validation events occur. Application callbacks are independent of settings in a vk_layer_settings.txt file which will be carried out separately. If no vk_layer_settings.txt file is present and no application callbacks are registered, error messages will be output through default logging callbacks.
Note that some layers are code-generated and will therefore exist in the directory
include/vkLayer.h - header file for layer code.
This layer emcompasses all of the functionality that used to be contained in the following layers: VK_LAYER_GOOGLE_threading, VK_LAYER_LUNARG_parameter_validation, VK_LAYER_LUNARG_object_tracker, VK_LAYER_LUNARG_core_validation, and VK_LAYER_GOOGLE_unique_objects. Each of these functional areas can still disabled individually, and are described below.
The object lifetime tracking will track object creation, use, and destruction. As objects are created their handles are stored in a data structure. As objects are used the layer verifies they exist in the data structure and output errors for unknown objects. As objects are destroyed they are removed from the data structure. At
vkDestroyInstance() times, if any objects have not been destroyed they are reported as leaked objects. If a debug callback function is registered this layer will use callback function(s) for reporting, otherwise it will use stdout.
The set of core checks does the bulk of the API validation that requires storing state. Some of the state it tracks includes the Descriptor Set, Pipeline State, Shaders, and dynamic state, and memory objects and bindings. It performs some point validation as states are created and used, and further validation Draw call and QueueSubmit time. Of primary interest is making sure that the resources bound to Descriptor Sets correctly align with the layout specified for the Set. Also, all of the image and buffer layouts are validated to make sure explicit layout transitions are properly managed. Related to memory, core_validation includes tracking object bindings, memory hazards, and memory object lifetimes. It also validates several other hazard-related issues related to command buffers, fences, and memory mapping. Additionally core_validation include shader validation (formerly separate shader_checker layer) that inspects the SPIR-V shader images and fixed function pipeline stages at PSO creation time. It flags errors when inconsistencies are found across interfaces between shader stages. The exact behavior of the checks depends on the pair of pipeline stages involved. If a debug callback function is registered, this layer will use callback function(s) for reporting, otherwise uses stdout. This layer also validates correct usage of image- and buffer-related APIs, including image and buffer parameters, formats, and correct use.
The stateless validation checks the input parameters to API calls for validity. If a debug callback function is registered, this layer will use callback function(s) for reporting otherwise uses stdout.
The thread-safety validation will check the multithreading of API calls for validity. Currently this checks that only one thread at a time uses an object in free-threaded API calls. If a debug callback function is registered, this layer will use callback function(s) for reporting, otherwise uses stdout.
The khronos layer framework also supports Vulkan handle wrapping. The Vulkan specification allows objects to have non-unique handles. This makes tracking object lifetimes difficult in that it is unclear which object is being referenced on deletion. When this functionalty is enabled (as it is by default) it will alias all objects with a unique object representation, allowing proper object lifetime tracking. This functionality may interfere with the development of proprietary Vulkan extension development, and is not strictly required for the proper operation of validation. One sign that it is needed is the appearance of errors emitted from the object_tracker layer indicating the use of previously destroyed objects.
Build VK loader using normal steps (cmake and make)
libVkLayer_khronos_validation.so in the same directory as your VK test or app:
cp build/layer/libVkLayer_khronos_validation.so build/tests
This is required for the Loader to be able to scan and enumerate your library. Alternatively, use the
VK_LAYER_PATH environment variable to specify where the layer libraries reside.
To specify how your layers should behave, create a vk_layer_settings.txt file. This file can exist in the same directory as your app or in a directory given by the
VK_LAYER_SETTINGS_PATH environment variable. Alternatively, you can use any filename you want if you set
VK_LAYER_SETTINGS_PATH to the full path of the file, rather than the directory that contains it.
Model the file after the following example: vk_layer_settings.txt
Specify which layers to activate using environment variables.
cd build/tests; ./vkinfo