Re: [Qemu-devel] Linux kernel polling for QEMU

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Nov 28, 2016 at 04:41:13PM +0100, Paolo Bonzini wrote:
> On 28/11/2016 16:29, Stefan Hajnoczi wrote:
> > Thanks for sharing the link.  I'll let you know before embarking on an
> > effort to make epoll support busy_loop.
> > 
> > At the moment I'm still evaluating whether the good results we've gotten
> > from polling in QEMU userspace are preserved when polling is shifted to
> > the kernel.
> > 
> > FWIW I've prototyped ioctl(EVENTFD_SET_POLL_INFO) but haven't had a
> > chance to test it yet:
> > https://github.com/stefanha/linux/commit/133e8f1da8eb5364cd5c5f7162decbc79175cd13
> 
> This would add a system call every time the main loop processes a vring,
> wouldn't it?

Yes, this is a problem and is the reason I haven't finished implementing
a test using QEMU yet.

My proposed eventfd polling mechanism doesn't work well with descriptor
ring indices because the polling info needs to be updated each event
loop iteration with the last seen ring index.

This can be solved by making struct eventfd_poll_info.value take a
userspace memory address.  The value to compare against is fetched each
polling iteration, avoiding the need for ioctl calls.

Stefan

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