From: Avi Kivity <avi@scylladb.com>
To: Paolo Bonzini <pbonzini@redhat.com>,
Stefan Hajnoczi <stefanha@gmail.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>,
linux-fsdevel@vger.kernel.org, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org,
Davide Libenzi <davidel@xmailserver.org>,
Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [RFC] eventfd: add EFD_AUTORESET flag
Date: Wed, 12 Feb 2020 12:54:30 +0200 [thread overview]
Message-ID: <cadb4320-4717-1a41-dfb5-bb782fd0a5da@scylladb.com> (raw)
In-Reply-To: <156cb709-282a-ddb6-6f34-82b4bb211f73@redhat.com>
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.
next prev parent reply other threads:[~2020-02-12 10:54 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-29 17:20 [RFC] eventfd: add EFD_AUTORESET flag Stefan Hajnoczi
2020-02-04 15:40 ` Stefan Hajnoczi
2020-02-11 9:32 ` Stefan Hajnoczi
2020-02-12 8:31 ` Paolo Bonzini
2020-02-12 10:10 ` Avi Kivity
2020-02-12 10:29 ` Stefan Hajnoczi
2020-02-12 10:47 ` Paolo Bonzini
2020-02-12 10:54 ` Avi Kivity [this message]
2020-02-19 10:37 ` Stefan Hajnoczi
2020-02-19 10:43 ` Avi Kivity
2020-02-19 11:10 ` Paolo Bonzini
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