linux-kernel - Re: [RFC] eventfd: add EFD

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Message-ID: <20200219103704.GA1076032@stefanha-x1.localdomain>
Date:   Wed, 19 Feb 2020 10:37:04 +0000
From:   Stefan Hajnoczi <stefanha@...il.com>
To:     Avi Kivity <avi@...lladb.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        linux-fsdevel@...r.kernel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Davide Libenzi <davidel@...ilserver.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC] eventfd: add EFD_AUTORESET flag

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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