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Date: Wed, 6 May 2015 20:20:23 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [RFC PATCH 6/7] virtio-blk: Don't handle output
 when there is "device IO" op blocker
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org, armbru@redhat.com, jcody@redhat.com, qemu-devel@nongnu.org, mreitz@redhat.com, Stefan Hajnoczi <stefanha@redhat.com>

On Wed, 05/06 14:07, Paolo Bonzini wrote:
> 
> 
> On 06/05/2015 13:23, Fam Zheng wrote:
> > virtio-blk now listens to op blocker change of the associated block
> > backend.
> > 
> > Up on setting op blocker on BLOCK_OP_TYPE_DEVICE_IO:
> > 
> >   non-dataplane:
> >    1) Set VirtIOBlock.paused
> >    2) In virtio_blk_handle_output, do nothing if VirtIOBlock.paused
> > 
> >   dataplane:
> >    1) Clear the host event notifier
> >    2) In handle_notify, do nothing if VirtIOBlock.paused
> > 
> > Up on removing the op blocker:
> > 
> >   non-dataplane:
> >    1) Clear VirtIOBlock.paused
> >    2) Schedule a BH on the AioContext of the backend, which calls
> >    virtio_blk_handle_output, so that the previous unhandled kicks can
> >    make progress
> > 
> >   dataplane:
> >    1) Set the host event notifier
> >    2) Notify the host event notifier so that unhandled events are
> >    processed
> > 
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> 
> Does non-dataplane need to do anything, since it uses iohandlers rather
> than aio_set_event_notifier_handler?

I guess it's not for this specific bug. See this as an attempt on a general
purpose "pause" mechanism to the device in investment for the future, for
example, bdrv_aio_drain. ;-)

I can drop it in v2 if you think the idea is not very helpful.

Fam