Adds an extension that allows applications to provide drivers to the loader during instance creation.

1. Problem Statement

There are several uses cases that cause applications to want to ship a driver with themselves. Adding a fallback CPU driver is necessary in many web browsers to support hardware which does not support vulkan, as well as for testing infrastructure where hardware might not be available. While there are currently several desktop Vulkan Loader mechanisms that allow applications to provide drivers, they all suffer from various shortcomings. A non comprehensive list is detailed below.

  • Some require an installer to be run. This necessitates an uninstaller, as well as makes the driver 'global' to the system or user.

  • They may require elevated privileges to use, such as with an installer, or causes the solution to fail when running with elevated privileges, as with Environment Variables.

2. Current Solution Space

  • Installation to OS specific locations

    1. Explicit installer and uninstaller needs to be run

    2. Global to all applications

    3. Requires elevated privileges in most cases

  • Environment Variables

    1. Disabled when running with elevated privileges due to the security concerns

    2. Tedious to setup since they require the full path to the driver manifest

  • MacOS Bundles - Allows placing the .dylib inside a relocatable package, loader knows how to look in them

    1. Not available for any other platform

  • Looking in current working directory

    1. Disabled when running with elevated privileges, as that is an attack vector

3. Proposal

VK_LUNARG_direct_driver_loading extends the pNext chain of VkInstanceCreateInfo to provide a list of structures which contain the information needed by the loader to load the drivers. This is achieved by providing the driver’s vkGetInstanceProcAddr, bypassing the loaders need to use the systems dynamic linker to load the drivers functions.

The extension also allows applications to use provided drivers exclusively, so that no drivers found on the system are loaded.

This satisfies the requirements:

  • Isolated from all other running processes.

  • No installation required.

  • Works when the application is running with elevated privileges.

The intent of the extension is to be implemented in the desktop Vulkan-Loader.

typedef struct VkDirectDriverLoadingInfoLUNARG {
    VkStructureType                         sType;
    const void*                             pNext;
    VkDirectDriverLoadingFlagsLUNARG        flags;
    PFN_vkGetInstanceProcAddr               pfnGetInstanceProcAddr;
} VkDirectDriverLoadingInfoLUNARG;

typedef struct VkDirectDriverLoadingListLUNARG {
    VkStructureType                         sType;
    const void*                             pNext;
    VkDirectDriverLoadingModeLUNARG         mode;
    uint32_t                                driverCount;
    const VkDirectDriverLoadingInfoLUNARG*  pDrivers;
} VkDirectDriverLoadingListLUNARG;

typedef enum VkDirectDriverLoadingModeLUNARG {
} VkDirectDriverLoadingModeLUNARG;

VkDirectDriverLoadingModeLUNARG allows applications to choose whether to load only the drivers they provide or include the drivers they provide with all of the drivers the Vulkan Loader finds on the system.

There is a known defect with this extension. When the application queries the list of instance extensions, there is no way to provide to the Vulkan Loader the list of application provided drivers, to combine the driver’s extensions with the extensions supported by the drivers and implicit layers on the system. As a workaround, applications can instead load the vkEnumerateInstanceExtensionProperties entrypoint from the provided drivers instead to get their list of extensions.

This workaround has a known defect when all of the following is true:

  1. An application uses VK_LUNARG_direct_driver_loading to add drivers.

  2. A layer (that will be enabled) filters out unsupported extensions during calls to vkEnumerateInstanceExtensionProperties entrypoint.

  3. The application enables instance extensions in VkInstanceCreateInfo which are supported by the application provided driver but not the layer(s).

As the application directly calls the driver’s vkEnumerateInstanceExtensionProperties instead of the loader’s, this prevents layers from being able to modify the list of extensions. Since the layer will not be able to filter out unsupported instance extensions, the layer may fail to work, cause bugs elsewhere, or crash.

4. Example Usage

This example shows typical usage where the provided driver should be the ONLY driver found. It uses VK_DIRECT_DRIVER_LOADING_MODE_EXCLUSIVE_LUNARG to prevent the loader from loading any drivers on the system, useful for use when running under testing conditions.

VkDirectDriverLoadingInfoLUNARG app_provided_driver{};
app_provided_driver.pfnGetInstanceProcAddr = /* address of drivers function pointer*/

VkDirectDriverLoadingListLUNARG direct_driver_loading{};
direct_driver_loading.mode = VK_DIRECT_DRIVER_LOADING_MODE_EXCLUSIVE_LUNARG; // Do not load any other drivers
direct_driver_loading.driverCount = 1;
direct_driver_loading.pDrivers = &app_provided_driver;

VkInstanceCreateInfo instance_info{};
instance_info.pNext = &direct_driver_loading;

vkCreateInstance(&instance_info, NULL, inst);

5. Issues

5.1. RESOLVED: Global functions are not extensible

Since this proposal allows adding drivers, and vkEnumerateInstanceExtensionProperties returns the list of extensions supported by drivers on the system, the application must either 'know' which extensions the provided drivers support or query them directly then manually enable the extensions in VkInstanceCreateInfo. While it is an adequate solution for most use cases, it does create a corner case when layers that modify the instance extension list are present. Fundamentally it is a problem with the design of extension enumeration and instance creation. While it is possible to add functionality to this extension which resolves the issue, it is better solved with its own extension since there are more issues with instance creation that need addressing than what this extension accomplishes.

5.2. RESOLVED: Should this extension also handle layer loading?

No, layers require significantly more information to be present for the loader to handle correctly, and have the same defect of global functions not being extensible.

5.3. RESOLVED: Do drivers implement this extension?

No, this would be implemented by the Vulkan Loader.

5.4. RESOLVED: Are there any changes drivers need to make to allow being used in this extension?

Partially, drivers do not need modification to work today. However, to support all of the features of the Loader-ICD interface, they will need to support the "Loader-ICD interface version 7". This version requires that all currently exported entry points in the Loader-ICD interface be queryable through vkGetInstanceProcAddr, which is simple addition.

5.5. RESOLVED: What kinds of drivers are expected to be used with this extension?

This extension is designed to be used with non-hardware drivers, such as software implementations like lavapipe and swiftshader, as well as API translation layers like MoltenVK. Trying to use this extension with hardware drivers is expressedly not intended and actively discouraged as hardware drivers require support by the Operating System and other system components that must be installed. Additionally the kernel components of hardware drivers are liable to change between versions rendering any user mode components useless after updates.