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 12/14/2018 05:56 PM, Dr. David Alan Gilbert wrote:
> > * 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?
> > 
> 
> I think normally, the source QEMU and destination QEMU should have the same
> QEMU booting parameter. If the destination QEMU doesn't support
> "--device virtio-balloon,free-page-hint=true", which the source QEMU has,
> the
> destination side QEMU will fail to boot, and migration will not happen then.

Ah that's OK; as long as free-page-hint is false by default that will
work fine.

Dave

> But I think there is still an option to make the migration possible. The
> "free-page-hint"
> can be set to false via e.g. QMP on the source side, then the destination
> side QEMU
> can boot without "free-page-hint".
> 
> Best,
> Wei
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK