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

Avi Kivity
2010-10-28T14:24:51Z

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.

--
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