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

[Stefan Hajnoczi]

On Mon, Nov 28, 2016 at 11:31:43AM +0200, Eliezer Tamir wrote:
> + Eric, Willem
> 
> On 24/11/2016 17:12, Stefan Hajnoczi wrote:
> > I looked through the socket SO_BUSY_POLL and blk_mq poll support in
> > recent Linux kernels with an eye towards integrating the ongoing QEMU
> > polling work.  The main missing feature is eventfd polling support which
> > I describe below.
> ...
> > State of polling in Linux
> > -------------------------
> > SO_BUSY_POLL causes recvmsg(2), select(2), and poll(2) family system
> > calls to spin awaiting new receive packets.  From what I can tell epoll
> > is not supported so that system call will sleep without polling.
> 
> At the time I sent out an RFC for epoll() SO_BUSY_POLL support.
> https://lkml.org/lkml/2013/8/21/192
> 
> In hindsight I think the way I tracked sockets was over-complicated.
> What I would do today would be to extend the API to allow the user
> to tell epoll which socket/queue combinations are interesting.
> 
> I would love to collaborate on this with you, though I must confess that
> my resources at the moment are limited and the setup I used for testing
> no longer exists.

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

Stefan

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