Support creating PhysicalDevice from raw value for OpenXR

[tmfink]

Summary

There are some problems in trying to use Vulkano with the OpenXR API (Rust bindings available via openxrs crate). The OpenXR API wants to create the Vulkan structs VkPhysicalDevice (instead of taking one that is already created).

The existing Vulkano APIs do not support taking in an externally created VkPhysicalDevice.

Details

Support for Vulkan in OpenXR comes via XR_KHR_vulkan_enable OpenXR extension.

The workflow to initialize OpenXR with Vulkan support (skipping some non-Vulkan details):

1. Get XrInstance from OpenXR by calling xrCreateInstance().
2. Get XrSystemId from OpenXR xrGetSystem()
3. Determine OpenXR runtime's Vulkan version info with XrGraphicsRequirementsVulkanKHR()
4. Determine OpenXR runtime's required Vulkan extensions with xrGetVulkanInstanceExtensionsKHR()
5. Get VkInstance from Vulkan by calling vkCreateInstance().
6. Get VkPhysicalDevice from OpenXR by calling xrGetVulkanGraphicsDeviceKHR().
   • Input: VkInstance
7. Get VkDevice from Vulkan by calling vkCreateDevice()
8. Get XrSession from OpenXR by calling xrCreateSession()
   • Inputs:
     • XrSessionCreateInfo
     • XrGraphicsBindingVulkanKHR (in next chain of XrSessionCreateInfo)
       • VkInstance instance
       • VkPhysicalDevice physicalDevice
       • VkDevice device;
       • uint32_t queueFamilyIndex
       • uint32_t queueIndex

According to XR_KHR_vulkan_enable spec:

physicalDevice VkPhysicalDevice must match the device specified by xrGetVulkanGraphicsDeviceKHR.

Vulkano cannot just create the the VkPhysicalDevice with Vulkan APIs.

Possible Solutions

1. Add an unsafe function to create vulkano::instance::PhysicalDevice from a raw VkPhysicalDevice.
   • This could be added through a new FromVulkanObject trait (opposite of the existing VulkanObject trait)
   • Downstream crates (such as openxrs) could do this for users to avoid exposing/requiring unsafe calls.
2. Call into OpenXR functions (such as xrGetVulkanGraphicsDeviceKHR()) to create a vulkano::instance::PhysicalDevice.
   • Would add a dependency (although optional) on OpenXR, which is non-trivial because it can be loaded in different ways

Thoughts? I'm not that familiar with Vulkan (or Vulkano).

[dyc3]

I was trying to do this, and ran into trouble when trying to create a session. I was unable to figure out how to provide any of these parameters, not just physical device.

[marc0246]

This was added in #1967, see also #1935 for the state of the overall effort.