Update (March 22nd @ 3:50pm EDT): And the Khronos Group has just responded to my follow-up questions. LDA has existed since Windows Vista, at the time for assisting with SLI and Crossfire support. Its implementation has changed in Windows 10, but that's not really relevant for Vulkan's multi-GPU support. To prove this, they showed LDA referenced in a Windows 8.1 MSDN post.

In short:

Vulkan's multi-GPU extensions can be used on Windows 7 and Windows 8.x. The exact process will vary from OS to OS, but the GPU vendor can implement these extensions if they choose, and LDA mode isn't exclusive to Windows 10.

 

Update (March 21st @ 11:55pm EDT): I came across a Microsoft Support page that discusses issues with LDA in Windows 7, so it seems like that functionality isn't limited to WDDM 2.0 and Windows 10. (Why have a support page otherwise?) Previously, I looked up an MSDN article that had it listed as a WDDM 2.0 feature, so I figured DSOGaming's assertion that it was introduced with WDDM 2.0 was correct.

As such, LDA might not require a GPU vendor's implementation at all. It'll probably be more clear when the Khronos Group responds to my earlier request, though.

That said, we're arguing over how much a GPU vendor needs to implement; either way, it will be possible to use the multi-GPU extensions in Windows 7 and Windows 8.x if the driver supports it.

Update (March 21st @ 7:30pm EDT): The Khronos Group has just released their statement. It's still a bit unclear, and I've submit another request for clarification.

Specifically, the third statement:

If an implementation on Windows does decide to use LDA mode, it is NOT tied to Windows 10. LDA mode has been available on many versions of Windows, including Windows 7 and 8.X.

… doesn't elaborate what is required for LDA mode on Windows outside of 10. (It could be Microsoft-supported, vendor-supported, or something else entirely.) I'll update again when that information is available. For now, it seems like the table, below, should actually look something like this:

  Implicit Multi-GPU
(LDA Implicit)		 Explicit Multi-GPU
(LDA Explicit)		 Unlinked Multi-GPU
(MDA)
Windows 7 Requires GPU Vendor
LDA Implementation?
(Or Equivalent)		 Requires GPU Vendor
LDA Implementation?
(Or Equivalent)		 ✓
Windows 8.1 Requires GPU Vendor
LDA Implementation?
(Or Equivalent)		 Requires GPU Vendor
LDA Implementation?
(Or Equivalent)		 ✓
Windows 10 ✓ ✓ ✓
macOS Apple doesn't allow the Vulkan API to ship in graphics drivers.
At all.
Linux / etc. ✓ ✓ ✓

… but we will update, again, should this be inaccurate.

Update (March 20th @ 3:50pm EDT): The Khronos Group has just responded that the other posts are incorrect. They haven't yet confirmed whether this post (which separates "device groups" from the more general "multi-GPU in Vulkan") is correct, though, because they're preparing an official statement. We'll update when we have more info.

Original Post Below (March 19th @ 9:36pm EDT)

A couple of days ago, some sites have noticed a bullet point that claims Windows-based GPU drivers will need WDDM in “linked display adapter” mode for “Native multi-GPU support for NVIDIA SLI and AMD Crossfire platforms” on Vulkan. This note came from an official slide deck by the Khronos Group, which was published during the recent Game Developers Conference, GDC 2017. The concern is that “linked display adapter” mode is a part of WDDM 2.0, which is exclusive to Windows 10.

This is being interpreted as “Vulkan does not support multi-GPU under Windows 7 or 8.x”.

I reached out to the Khronos Group for clarification, but I’m fairly sure I know what this does (and doesn’t) mean. Rather than starting with a written out explanation in prose, I will summarize it into a table, below, outlining what is possible on each platform. I will then elaborate below that.

  Implicit Multi-GPU
(LDA Implicit)		 Explicit Multi-GPU
(LDA Explicit)		 Unlinked Multi-GPU
(MDA)
Windows 7     ✓
Windows 8.1     ✓
Windows 10 ✓ ✓ ✓
macOS Apple doesn't allow the Vulkan API to ship in graphics drivers.
At all.
Linux / etc. ✓ ✓ ✓

So the good news is that it’s possible for a game developer to support multi-GPU (through what DirectX 12 would call MDA) on Windows 7 and Windows 8.x; the bad news is that no-one might bother with the heavy lifting. Linked display adapters allow the developer to assume that all GPUs are roughly the same performance, have the same amount of usable memory, and can be accessed through a single driver interface. On top of these assumptions, device groups also hide some annoying and tedious work inside the graphics driver, like producing a texture on one graphics card and quickly giving it to another GPU for rendering.

Basically, if the developer will go through the trouble of supporting AMD + NVIDIA or discrete GPU + integrated GPU systems, then they can support Windows 7 / 8.x in multi-GPU as well. Otherwise? Your extra GPUs will be sitting out unless you switch to DirectX 11 or OpenGL (or you use it for video encoding or something else outside the game).

On the other hand, this limitation might pressure some developers to support unlinked multi-GPU configurations. There are some interesting possibilities, including post-processing, GPGPU tasks like AI visibility and physics, and so forth, which might be ignored in titles whose developers were seduced by the simplicity of device groups. On the whole, device groups was apparently a high-priority request by game developers, and its inclusion will lead to more multi-GPU content. Developers who can justify doing it themselves, though, now have another reason to bother re-inventing a few wheels.

Or… you could just use Linux. That works, too.

Again, we are still waiting on the Khronos Group to confirm this story. See the latest update, above.