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

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Date:   Wed, 12 Feb 2020 11:47:45 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     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, Avi Kivity <avi@...lladb.com>,
        Davide Libenzi <davidel@...ilserver.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC] eventfd: add EFD_AUTORESET flag

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.

Paolo

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