In Vulkan how can you associate each individual video card with monitors they're directly connected to

Question

I have two monitors, each connected to a different GPU. Both GPUs are in a single machine, and I want to run a single application. I have two independent views, and I would like to render each one using a GPU/Monitor set. I can create multiple surfaces and devices, but I want to ensure I associate each surface with the GPU its monitor is plugged into, otherwise I suspect I'll suffer performance issues as the frame buffers need to be copied back and forth between cards.

I'm using fullscreen surfaces, and I was thinking this was something vkGetPhysicalDeviceSurfaceSupportKHR would tell me. However, both VkSurfaceKHR appear to be valid targets for each VkPhysicalDevice so I guess this is something the OS and GPU Driver can handle, but is there any hint about which surface is optimal to associate with a device?

From what I can tell the extension VK_KHR_display is one way of doing this, but it's not available on my Windows 10 machine or Nvidia GPU. It seems to be intended for embedded platforms only. However it lets you list attached displays for each device which is pretty much what I'm looking for: https://vulkan.lunarg.com/doc/view/1.0.30.0/linux/vkspec.chunked/ch29s03.html

This quote from the docs makes me belive this may not be supported on Windows:

Issues

1) Does Win32 need a way to query for compatibility between a particular physical device and a specific screen? Compatibility between a physical device and a window generally only depends on what screen the window is on. However, there is not an obvious way to identify a screen without already having a window on the screen.

RESOLVED: No. While it may be useful, there is not a clear way to do this on Win32. However, a method was added to query support for presenting to the windows desktop as a whole.

However, I'm still interested in hearing if there's a work around to achieve a similar effect.

Answer 1

Finally figured out a work around for this:

Direct X actually supports this through use of the IDXGIAdapter::EnumOutputs function. This lets you list the monitors connected to each GPU. Then using these two extensions you can remap this information to Vulkan:

• VK_KHR_external_memory_capabilities
• VK_KHR_get_physical_device_properties2

You can use these to get the deviceLUID from VkPhysicalDeviceIDPropertiesKHR. This can then be compared with the Luid from this structure in Direct X DXGI_ADAPTER_DESC

You can also use glfwGetWin32Window to get the HWND of the monitor. This lets you associate a vulkan surface with a direct x monitor.

You now have all the information you need to accociate vulkan surfaces with the devices they're actually connected to.

At least in my application, setting this up correctly results in a significant difference in performance.

This would all be way simpler (and cross platform) if Windows would just support the VK_KHR_display and VK_KHR_display_swapchain extensions as Linux does.