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Re: [Qemu-devel] [PATCH] Use qemu_eventfd for POSIX AIO

From: Jan Kiszka
Subject: Re: [Qemu-devel] [PATCH] Use qemu_eventfd for POSIX AIO
Date: Tue, 27 Sep 2011 16:22:45 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv: Gecko/20080226 SUSE/ Thunderbird/ Mnenhy/

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.


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Corporate Competence Center Embedded Linux

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