linux-kernel - Re: [Qemu-devel] Re: [PATCH] Implement a virtio GPU transport

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Date:	Thu, 28 Oct 2010 20:50:27 +0100
From:	Ian Molton <ian.molton@...labora.co.uk>
To:	Anthony Liguori <anthony@...emonkey.ws>
CC:	Avi Kivity <avi@...hat.com>, virtualization@...ts.osdl.org,
	linux-kernel@...r.kernel.org,
	QEMU Developers <qemu-devel@...gnu.org>
Subject: Re: [Qemu-devel] Re: [PATCH] Implement a virtio GPU transport

On 28/10/10 15:43, Anthony Liguori wrote:
> On 10/28/2010 09:24 AM, Avi Kivity wrote:
>> On 10/28/2010 01:54 PM, Ian Molton wrote:
>
>>> True, but then all that would prove is that I can write a spec to
>>> match the code.
>>
>> It would also allow us to check that the spec matches the
>> requirements. Those two steps are easier than checking that the code
>> matches the requirements.

There was no formal spec for this. The code was written to replace nasty 
undefined-instruction based data transport hacks in the (already 
existing) GL passthrough code.

> I'm extremely sceptical of any GL passthrough proposal. There have
> literally been half a dozen over the years and they never seem to leave
> proof-of-concept phase. My (limited) understanding is that it's a
> fundamentally hard problem that no one has adequately solved yet.

The code in this case has been presented as a patch to qemu nearly 3 
years ago. I've taken the patches and refactored them to use virtio 
rather than an undefined instruction (which fails under KVM, unlike my 
approach).

Its in use testing meego images and seems to be fairly reliable. it can 
handle compositing window managers, games, video, etc. We're currently 
supporting OpenGL 1.4 including shaders.

> A specifically matters an awful lot less than an explanation of how the
> problem is being solved in a robust fashion such that it can be reviewed
> by people with a deeper understanding of the problem space.

I'm not sure there is a way to prevent nefarious tasks upsetting the 
hosts OpenGL with carefully crafted strings of commands, short of 
inspecting every single command, which is insane.

Really this needs to be done at a lower level by presenting a virtual 
GPU to the guest OS but I am not in a position to code that right now.

The code as it is is useful, and can always be superceeded by a virtual 
GPU implementation in future.

At least this breaks the chicken / egg cycle of people wanting GL 
support on virtual machines, but not writing stuff to take advantage of 
it because the support isn't there. its also a neatly encapsulated 
solution - if you dont want people to have access to the passthrough, 
simply tell qemu not to present the virtio-gl device to the guest, via 
qemus existing commandline options.

If this code was invasive to qemus core, I'd say 'no way' but its just 
not. and as the GL device is versioned, we can keep using it even if the 
passthrough is replaced by a virtual GPU.
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