Re: [RFC] eventfd: add EFD_AUTORESET flag

From: Avi Kivity
Date: Wed Feb 19 2020 - 05:43:58 EST

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
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.