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



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