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

Anthony Liguori
2010-10-28T14:43:47Z

On 10/28/2010 09:24 AM, Avi Kivity wrote:
> 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.

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.

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.

Regards,

Anthony Liguori

>> 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.
>