Re: [Qemu-devel] [PATCH v11 7/7] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

[Dr. David Alan Gilbert]

* Wei Wang (address@hidden) wrote:
> On 12/13/2018 11:45 PM, Dr. David Alan Gilbert wrote:
> > * Wei Wang (address@hidden) wrote:
> > > The new feature enables the virtio-balloon device to receive hints of
> > > guest free pages from the free page vq.
> > > 
> > > A notifier is registered to the migration precopy notifier chain. The
> > > notifier calls free_page_start after the migration thread syncs the dirty
> > > bitmap, so that the free page optimization starts to clear bits of free
> > > pages from the bitmap. It calls the free_page_stop before the migration
> > > thread syncs the bitmap, which is the end of the current round of ram
> > > save. The free_page_stop is also called to stop the optimization in the
> > > case when there is an error occurred in the process of ram saving.
> > > 
> > > Note: balloon will report pages which were free at the time of this call.
> > > As the reporting happens asynchronously, dirty bit logging must be
> > > enabled before this free_page_start call is made. Guest reporting must be
> > > disabled before the migration dirty bitmap is synchronized.
> > > 
> > > Signed-off-by: Wei Wang <address@hidden>
> > > CC: Michael S. Tsirkin <address@hidden>
> > > CC: Dr. David Alan Gilbert <address@hidden>
> > > CC: Juan Quintela <address@hidden>
> > > CC: Peter Xu <address@hidden>
> > I think I'm OK for this from the migration side, I'd appreciate
> > someone checking the virtio and aio bits.
> > 
> > I'm not too sure how it gets switched on and off - i.e. if we get a nice
> > new qemu on a new kernel, what happens when I try and migrate to the
> > same qemu on an older kernel without these hints?
> > 
> 
> This feature doesn't rely on the host kernel. Those hints are reported from
> the guest kernel.
> So migration across different hosts wouldn't affect the use of this feature.
> Please correct me if I didn't get your point.

Ah OK, yes;  now what about migrating from new->old qemu with a new
guest but old machine type?

Dave

> Best,
> Wei
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK