* Wei Wang (wei.w.w...@intel.com) wrote: > On 12/14/2018 05:56 PM, Dr. David Alan Gilbert wrote: > > * Wei Wang (wei.w.w...@intel.com) wrote: > > > On 12/13/2018 11:45 PM, Dr. David Alan Gilbert wrote: > > > > * Wei Wang (wei.w.w...@intel.com) 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 <wei.w.w...@intel.com> > > > > > CC: Michael S. Tsirkin <m...@redhat.com> > > > > > CC: Dr. David Alan Gilbert <dgilb...@redhat.com> > > > > > CC: Juan Quintela <quint...@redhat.com> > > > > > CC: Peter Xu <pet...@redhat.com> > > > > 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 / dgilb...@redhat.com / Manchester, UK
