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Date: Tue, 27 Sep 2011 16:22:45 +0200
From: Jan Kiszka <jan.kiszka@siemens.com>
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Subject: Re: [Qemu-devel] [PATCH] Use qemu_eventfd for POSIX AIO
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Avi Kivity <avi@redhat.com>

On 2011-09-27 16:19, Anthony Liguori wrote:
> On 09/27/2011 09:11 AM, Avi Kivity wrote:
>> On 09/27/2011 05:07 PM, Anthony Liguori wrote:
>>>
>>> You don't expect to handle EAGAIN with eventfd() whereas you have to handle it
>>> with pipe().
>>>
>>> Moreover, the eventfd() counter is not lossy (practically speaking) whereas if
>>> you use pipe() as a counter, it will be lossy in practice.
>>>
>>> This is why posix aio uses pipe() and not eventfd().
>>
>> We could define a qemu_event mechanism that satisfies the least common
>> denominator, and is implemented by eventfd when available.
>>
>> qemu_event_create()
>> qemu_event_signal()
>> qemu_event_wait()
>> qemu_event_poll_add() // registers in main loop
>> qemu_event_poll_del()
> 
> See hw/event_notifier.[ch].

That code looks suspicious, btw. It claims things ("we use
EFD_SEMAPHORE") it does not do.

Jan

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