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Date: Wed, 16 Mar 2016 18:51:17 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/4] block: Use drained section in
	bdrv_set_aio_context
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>, qemu-devel@nongnu.org, tubo@linux.vnet.ibm.com, Stefan Hajnoczi <stefanha@redhat.com>, cornelia.huck@de.ibm.com, borntraeger@de.ibm.com

On Wed, 03/16 11:27, Paolo Bonzini wrote:
> 
> 
> On 16/03/2016 11:10, Fam Zheng wrote:
> > An empty begin/end pair is almost the same as a bare bdrv_drain except
> > the aio_poll inside is wrapped by
> > aio_disable_external/aio_enable_external.
> > 
> > This is safer, and is the only way to achieve quiescence in this
> > aio_poll(), because bdrv_drained_begin/end pair cannot span across
> > context detach/attach options, so it's not possible to do by the caller.
> 
> I'm still not sure about this patch.
> 
> When starting dataplane, the ioeventfd is registered with iohandler.c so
> bdrv_drained_begin/end is not necessary.

You are right, and looks like the k->set_host_notifier() above
blk_set_aio_context would disable the fd anyway.

> 
> Likewise when stopping dataplane bdrv_set_aio_context is called after
> the thread has been stopped and thus the ioeventfd is not registered
> anymore as an external client.

Right.

Fam