This document proposes an extension to allow the loader to understand driver layering, for improving physical device sorting.

1. Problem Statement

The Vulkan loader is able to sort physical devices according to platform-specific criteria. For example, on Windows, the loader uses LUIDs to put physical devices in the same order as DXGI adapters. However, it is possible to have multiple Vulkan drivers that provide support for the same physical device, for example, where one is a “native” vendor-provided implementation and another is a “layered” implementation on top of a different API. Examples of layered implementations would include VulkanOn12 (aka Dozen), layered on D3D12, and MoltenVK, layered on Metal.

On a system where a physical device has two possible drivers, the sort order between them is currently unspecified. An ideal sort order should place any native/un-layered drivers sorted-before any layered drivers, as it should be expected that native drivers will provide more functionality and higher performance, since layering inherently adds overhead. But the loader has no way of knowing which driver to prefer.

An additional problem that is not addressed by this specification is the case where you have multiple “native” drivers for a single physical device. In that case, the sort order remains unspecified, as a correct ordering between drivers is non-obvious.

2. Solution Space

Options that were considered include: * Special-casing well-known layered drivers in the Vulkan loader. * Extending the Loader-ICD interface to identify layered drivers. * Providing an extension to allow layered drivers to self-identify.

The latter solution is the more general, and also has the benefit of allowing applications to understand when they are running on a layered driver.

3. Proposal

The following properties are exposed by the VK_MSFT_layered_driver extension:

typedef enum VkLayeredDriverUnderlyingApiMSFT {
} VkLayeredDriverUnderlyingApiMSFT;

typedef struct VkPhysicalDeviceLayeredDriverPropertiesMSFT {
    VkStructureType sType;
    const void* pNext;
    VkLayeredDriverUnderlyingApiMSFT underlyingAPI;
} VkPhysicalDeviceLayeredDriverPropertiesMSFT;

Layered drivers should implement this extension. The Vulkan loader can then be updated to query for the this structure. If the underlyingAPI is not NONE, then the driver should be considered layered. The specific value of underlyingAPI is simply informational for applications to query if they so choose.

4. Issues

4.1. RESOLVED: Is a string the right way to identify an underlying API?

No, an enum is a much better solution. The same conclusion was already reached with the VkDriverId enum.

5. Further Functionality

Additional properties of the layering implementation, such as underlying API object pointers, could be exposed, but considering that the nature of those will depend on the underlying API, it seems like those should be exposed via separate extensions, if at all.

It might make sense to add things like driver version for the underlying driver, since the version information exposed through existing properties would refer to the version of layered implementation.