Re: [RFC PATCH 0/4] DirectX on Linux

Dave Airlie
2020-05-19T23:12:40Z

On Wed, 20 May 2020 at 08:42, Dave Airlie <airlied@gmail.com> wrote:
>
> On Wed, 20 May 2020 at 02:33, Sasha Levin <sashal@kernel.org> wrote:
> >
> > There is a blog post that goes into more detail about the bigger
> > picture, and walks through all the required pieces to make this work. It
> > is available here:
> > https://devblogs.microsoft.com/directx/directx-heart-linux . The rest of
> > this cover letter will focus on the Linux Kernel bits.
> >
> > Overview
> > ========
> >
> > This is the first draft of the Microsoft Virtual GPU (vGPU) driver. The
> > driver exposes a paravirtualized GPU to user mode applications running
> > in a virtual machine on a Windows host. This enables hardware
> > acceleration in environment such as WSL (Windows Subsystem for Linux)
> > where the Linux virtual machine is able to share the GPU with the
> > Windows host.
> >
> > The projection is accomplished by exposing the WDDM (Windows Display
> > Driver Model) interface as a set of IOCTL. This allows APIs and user
> > mode driver written against the WDDM GPU abstraction on Windows to be
> > ported to run within a Linux environment. This enables the port of the
> > D3D12 and DirectML APIs as well as their associated user mode driver to
> > Linux. This also enables third party APIs, such as the popular NVIDIA
> > Cuda compute API, to be hardware accelerated within a WSL environment.
> >
> > Only the rendering/compute aspect of the GPU are projected to the
> > virtual machine, no display functionality is exposed. Further, at this
> > time there are no presentation integration. So although the D3D12 API
> > can be use to render graphics offscreen, there is no path (yet) for
> > pixel to flow from the Linux environment back onto the Windows host
> > desktop. This GPU stack is effectively side-by-side with the native
> > Linux graphics stack.
>
> Okay I've had some caffiene and absorbed some more of this.
>
> This is a driver that connects a binary blob interface in the Windows
> kernel drivers to a binary blob that you run inside a Linux guest.
> It's a binary transport between two binary pieces. Personally this
> holds little of interest to me, I can see why it might be nice to have
> this upstream, but I don't forsee any other Linux distributor ever
> enabling it or having to ship it, it's purely a WSL2 pipe. I'm not
> saying I'd be happy to see this in the tree, since I don't see the
> value of maintaining it upstream, but it probably should just exists
> in a drivers/hyperv type area.
>
> Having said that, I hit one stumbling block:
> "Further, at this time there are no presentation integration. "
>
> If we upstream this driver as-is into some hyperv specific place, and
> you decide to add presentation integration this is more than likely
> going to mean you will want to interact with dma-bufs and dma-fences.
> If the driver is hidden away in a hyperv place it's likely we won't
> even notice that feature landing until it's too late.
>
> I would like to see a coherent plan for presentation support (not
> code, just an architectural diagram), because I think when you
> contemplate how that works it will change the picture of how this
> driver looks and intergrates into the rest of the Linux graphics
> ecosystem.
>
> As-is I'd rather this didn't land under my purview, since I don't see
> the value this adds to the Linux ecosystem at all, and I think it's
> important when putting a burden on upstream that you provide some
> value.

I also have another concern from a legal standpoint I'd rather not
review the ioctl part of this. I'd probably request under DRI
developers abstain as well.

This is a Windows kernel API being smashed into a Linux driver. I
don't want to be tainted by knowledge of an API that I've no idea of
the legal status of derived works. (it this all covered patent wise
under OIN?)

I don't want to ever be accused of designing a Linux kernel API with
illgotten D3DKMT knowledge, I feel tainting myself with knowledge of a
properietary API might cause derived work issues.

Dave.