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. 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. Stefan
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