Re: [Qemu-devel] [PATCH v6 11/12] qed: Implement .bdrv_drain

[Fam Zheng <famz@redhat.com>]

On Thu, 10/22 22:59, Paolo Bonzini wrote:
> 
> 
> On 22/10/2015 12:53, Kevin Wolf wrote:
> > Am 22.10.2015 um 08:32 hat Fam Zheng geschrieben:
> >> The "need_check_timer" is used to clear the "NEED_CHECK" flag in the
> >> image header after a grace period once metadata update has finished. In
> >> compliance to the bdrv_drain semantics we should make sure it remains
> >> deleted once .bdrv_drain is called.
> >>
> >> We cannot reuse qed_need_check_timer_cb because it calls bdrv_aio_flush
> >> with a completion callback that starts more I/O, bdrv_drain cannot cope
> >> with this.
> > 
> > For the record: I discussed this with Fam on IRC and we came to the
> > conclusion that we should instead fix bdrv_drain() to correctly drain
> > in-flight flushes.
> > 
> > In order to achieve this, Fam will send a series that adds a
> > BdrvTrackedRequest to all remaining asynchronous operations. So far we
> > have identified flush, discard and aio_ioctl. At the same time he will
> > send a rebased version of this series that goes back to asynchronous
> > flushing in bdrv_qed_drain().
> 
> This is a completely separate bug. Can you at least merge all patches
> except this, so that everything except QED can do snapshots with
> dataplane?  Of course there will be a v7 at least for patch 9.
> 

So I'll drop the two patches for QED in v7, and let them follow the bdrv_drain
fixing series.

Actually, I think the current being of the QED timer doesn't change guest
visible data - if the callback would dequeue more allocating requests
(qed_unplug_allocating_write_reqs), the loop in bdrv_drain will wait for them.
So it's not a big problem for the sake of transaction after all.

Fam