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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 16:45:32 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv: Gecko/20080226 SUSE/ Thunderbird/ Mnenhy/

On 2011-09-27 16:42, Avi Kivity wrote:
> On 09/27/2011 05:36 PM, Jan Kiszka wrote:
>> On 2011-09-27 16:34, Avi Kivity wrote:
>>>  On 09/27/2011 05:29 PM, Jan Kiszka wrote:
>>>>>   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().
>>>>  I don't get this yet. eventfd is lossy by default. It only decreases the
>>>>  counter on read if you specify EFD_SEMAPHORE - which we do not do.
>>>  It's not lossy - a read returns the number of events written since the
>>>  last read.
>> Yeah, but what's the point? We don't evaluate this.
> If we write an interface that looks like eventfd but subtly differs, 
> someone will get bitten.  If we write a new interface and implement it 
> via eventfd (or a pipe), no one gets bitten.

I don't disagree that there is still room for improving the existing
interface, generalizing to qemu_event. But that's not in the scope of
this patch.


Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

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