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

[Wang, Wei W]

On Friday, December 14, 2018 7:17 PM, Dr. David Alan Gilbert 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
> 

Hi Dave,

Could we have this feature in QEMU 4.0 (freeze on Mar 12)?

Best,
Wei