Re: [Qemu-devel] [PATCH v2] balloon: Fix failure of updating guest memory status

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Aug 01, 2016 at 11:59:31PM +0000, Li, Liang Z wrote:
> > On Wed, Jul 06, 2016 at 12:49:06PM +0000, Li, Liang Z wrote:
> > > > > > > After live migration, 'guest-stats' can't get the expected
> > > > > > > memory status in the guest. This issue is caused by commit
> > 4eae2a657d.
> > > > > > > The value of 's->stats_vq_elem' will be NULL after live
> > > > > > > migration, and the check in the function
> > > > > > > 'balloon_stats_poll_cb()' will prevent the 'virtio_notify()'
> > > > > > > from executing. So guest will not update the memory status.
> > > > > > >
> > > > > > > Commit 4eae2a657d is doing the right thing, but 's->stats_vq_elem'
> > > > > > > should be treated as part of balloon device state and migrated
> > > > > > > to destination if it's not NULL to make everything works well.
> > > > > > >
> > > > > > > Signed-off-by: Liang Li <liang.z.li@intel.com>
> > > > > > > Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> > > > > > > Cc: Michael S. Tsirkin <mst@redhat.com>
> > > > > > > Cc: Ladi Prosek <lprosek@redhat.com>
> > > > > > > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > > > > >
> > > > > > I agree there's an issue but we don't change versions anymore.
> > > > > > Breaking migrations for everyone is also not nice.
> > > > > >
> > > > > > How about queueing virtio_balloon_receive_stats so it will get
> > > > > > invoked when vm starts?
> > > > > >
> > > > >
> > > > > Could you give more explanation about how it works?  I can't catch you.
> > > > >
> > > > > Thanks!
> > > > > Liang
> > > >
> > > > virtqueue_discard before migration
> > > >
> > > > virtio_balloon_receive_stats after migration
> > > >
> > >
> > > Sorry, I still can't catch you. Maybe it's easier for you to submit a
> > > patch than writing a lot a words to make me understand your idea.
> > 
> > I'm rather busy now.  I might look into it towards end of the month.
> > 
> > > I just don't understand why not to use the version to make things
> > > easier, is that not the original intent of version id?
> > 
> > This was the original idea but we stopped using version ids since they have
> > many shortcomings.
> > 
> > > If we want to extend the device and more states are needed, the idea
> > > you suggest can be used as a common solution?
> > >
> > > Thanks!
> > > Liang
> > 
> > The idea is to try to avoid adding more state. that's not always possible but in
> > this case element was seen but not consumed yet, so it should be possible
> > for destination to simply get it from the VQ again.
> > 
> > > > --
> > > > MST
> 
> Hi Michel,
> 
> Do you have time for this issue recently?  
> 
> Thanks!
> Liang


Sorry, doesn't look like I will.
Idea is to make sure balloon_stats_poll_cb runs
on source. This will set stats_vq_elem to NULL.


-- 
MST