Re: [RFC] eventfd: add EFD_AUTORESET flag

[Avi Kivity]

On 19/02/2020 12.37, Stefan Hajnoczi wrote:
> On Wed, Feb 12, 2020 at 12:54:30PM +0200, Avi Kivity wrote:
> > On 12/02/2020 12.47, Paolo Bonzini wrote:
> > > On 12/02/20 11:29, Stefan Hajnoczi wrote:
> > > > On Wed, Feb 12, 2020 at 09:31:32AM +0100, Paolo Bonzini wrote:
> > > > > On 29/01/20 18:20, Stefan Hajnoczi wrote:
> > > > > > +	/* Semaphore semantics don't make sense when autoreset is enabled */
> > > > > > +	if ((flags & EFD_SEMAPHORE) && (flags & EFD_AUTORESET))
> > > > > > +		return -EINVAL;
> > > > > > +
> > > > > I think they do, you just want to subtract 1 instead of setting the
> > > > > count to 0.  This way, writing 1 would be the post operation on the
> > > > > semaphore, while poll() would be the wait operation.
> > > > True!  Then EFD_AUTORESET is not a fitting name.  EFD_AUTOREAD or
> > > > EFD_POLL_READS?
> > > Avi's suggestion also makes sense.  Switching the event loop from poll()
> > > to IORING_OP_POLL_ADD would be good on its own, and then you could make
> > > it use IORING_OP_READV for eventfds.
> > >
> > > In QEMU parlance, perhaps you need a different abstraction than
> > > EventNotifier (let's call it WakeupNotifier) which would also use
> > > eventfd but it would provide a smaller API.  Thanks to the smaller API,
> > > it would not need EFD_NONBLOCK, unlike the regular EventNotifier, and it
> > > could either set up a poll() handler calling read(), or use
> > > IORING_OP_READV when io_uring is in use.
> > Just to be clear, for best performance don't use IORING_OP_POLL_ADD, just
> > IORING_OP_READ. That's what you say in the second paragraph but the first
> > can be misleading.


Actually it turns out that current uring OP_READ throws the work into a 
workqueue. Jens is fixing that now.


> Thanks, that's a nice idea!  I already have experimental io_uring fd
> monitoring code written for QEMU and will extend it to use IORING_OP_READ.


Note linux-aio can do IOCB_CMD_POLL, starting with 4.19.