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

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Date:	Thu, 28 Oct 2010 16:24:04 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Ian Molton <ian.molton@...labora.co.uk>
CC:	linux-kernel@...r.kernel.org,
	QEMU Developers <qemu-devel@...gnu.org>,
	virtualization@...ts.osdl.org
Subject: Re: [Qemu-devel] Re: [PATCH] Implement a virtio GPU transport

  On 10/28/2010 01:54 PM, Ian Molton wrote:
>> Well, I like to review an implementation against a spec.
>
>
> 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.

> The code is proof of concept. the kernel bit is pretty simple, but I'd 
> like to get some idea of whether the rest of the code will be accepted 
> given that theres not much point in having any one (or two) of these 
> components exist without the other.

I guess some graphics people need to be involved.

>
>> Better, but still unsatisfying. If the server is busy, the caller would
>> block. I guess it's expected since it's called from ->fsync(). I'm not
>> sure whether that's the best interface, perhaps aio_writev is better.
>
> The caller is intended to block as the host must perform GL rendering 
> before allowing the guests process to continue.

Why is that?  Can't we pipeline the process?

>
> The only real bottleneck is that processes will block trying to submit 
> data if another process is performing rendering, but that will only be 
> solved when the renderer is made multithreaded. The same would happen 
> on a real GPU if it had only one queue too.
>
> If you look at the host code, you can see that the data is already 
> buffered per-process, in a pretty sensible way. if the renderer itself 
> were made a seperate thread, then this problem magically disappears 
> (the queuing code on the host is pretty fast).

Well, this is out of my area of expertise.  I don't like it, but if it's 
acceptable to the gpu people, okay.

>
> In testing, the overhead of this was pretty small anyway. Running a 
> few dozen glxgears and a copy of ioquake3 simultaneously on an intel 
> video card managed the same framerate with the same CPU utilisation, 
> both with the old code and the version I just posted. Contention 
> during rendering just isn't much of an issue.

-- 
error compiling committee.c: too many arguments to function

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