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

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Date:   Wed, 12 Feb 2020 12:54:30 +0200
From:   Avi Kivity <avi@...lladb.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Stefan Hajnoczi <stefanha@...il.com>
Cc:     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 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.