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From:   Dave Airlie <airlied@gmail.com>
Date:   Wed, 20 May 2020 08:42:43 +1000
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Subject: Re: [RFC PATCH 0/4] DirectX on Linux
To:     Sasha Levin <sashal@kernel.org>
Cc:     "Deucher, Alexander" <alexander.deucher@amd.com>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= <ville.syrjala@linux.intel.com>,
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        "Ursulin, Tvrtko" <tvrtko.ursulin@intel.com>,
        linux-hyperv@vger.kernel.org, sthemmin@microsoft.com,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        haiyangz@microsoft.com, LKML <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        spronovo@microsoft.com, wei.liu@kernel.org,
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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.

Dave.