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Date: Fri, 12 Apr 2013 14:22:08 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [RFC 00/13] aio: drop io_flush()
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Liu Ping Fan <pingfank@linux.vnet.ibm.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>

Am 12.04.2013 um 14:06 hat Stefan Hajnoczi geschrieben:
> On Fri, Apr 12, 2013 at 12:27 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > Il 12/04/2013 12:04, Kevin Wolf ha scritto:
> >> Or actually, if you want to avoid .bdrv_drain for now, the patch that I
> >> started when I thought a bit about this, had a default .bdrv_drain
> >> implementation that just forwarded the request to bs->file if it wasn't
> >> implemented by a block driver. For the QED case, this would work.
> >
> > It's similar to bdrv_co_flush.  Drain bs first, then bs->backing_hd
> > (this is not needed in bdrv_co_flush), then bs->file, then the driver
> > can do it on other files.
> 
> Thanks for explaining Kevin.  I didn't check whether bdrv_states
> included all BlockDriverStates.
> 
> The simplest solution is to put all BlockDriverStates on a global list.
> 
> A .bdrv_drain() interface is more flexible but I don't see a need for it yet.

We don't need the .bdrv_drain() callback yet, but please implement it
without a global list and with a function that forwards requests to
bs->file and bs->backing_hd like Paolo suggested. This will be the
obvious place to add the .bdrv_drain() callback later.

The reason why I prefer this approach is that it allows draining a
single BlockDriverState. After your patches we're really close to
enabling this and converting some bdrv_drain_all() to more specific
bdrv_drain(bs) calls.

Kevin