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Date: Tue, 27 Sep 2011 09:38:25 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH] Use qemu_eventfd for POSIX AIO
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To: Jan Kiszka <jan.kiszka@siemens.com>
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 09/27/2011 09:22 AM, Jan Kiszka wrote:
> 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.

Indeed.

But the interface appears to be the right one IMHO.

Regards,

Anthony Liguori

>
> Jan
>