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Date: Tue, 27 Sep 2011 16:41:44 +0200
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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>, Jan Kiszka <jan.kiszka@siemens.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Avi Kivity <avi@redhat.com>

On 09/27/2011 04:07 PM, Anthony Liguori wrote:
>
> I think it's a bit dangerous to implement eventfd() in terms of pipe().
>
> 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().

But this is the same idiom we use for the iothread signaling.  We're not 
using the eventfd's counter.  Perhaps it would be nice to complete 
EventNotifier with "notify event" methods and use it, but Jan's patch is 
safe, I think.

Paolo