On Wed, Dec 12, 2012 at 06:39:15PM +0100, Paolo Bonzini wrote: > Il 12/12/2012 18:14, Michael S. Tsirkin ha scritto: > > On Wed, Dec 12, 2012 at 05:51:51PM +0100, Paolo Bonzini wrote: > >> Il 12/12/2012 17:37, Michael S. Tsirkin ha scritto: > >>>> You wrote "the only way to know head 1 is outstanding is because backend > >>>> has stored this info somewhere". But the backend _is_ tracking it (by > >>>> serializing and then restoring the VirtQueueElement) and no leak happens > >>>> because virtqueue_fill/flush will put the head on the used ring sooner > >>>> or later. > >>> > >>> If you did this before save vm inuse would be 0. > >> > >> No, I won't. I want a simple API that the device can call to keep inuse > >> up-to-date. Perhaps a bit ugly compared to just saving inuse, but it > >> works. Or are there other bits that need resyncing besides inuse? Bits > >> that cannot be recovered from the existing migration data? > > > > Saving inuse counter is useless. We need to know which requests > > are outstanding if we want to retry them on remote. > > And that's what virtio-blk and virtio-scsi have been doing for years.
I don't see it - all I see in save is virtio_save. there's the extra code to save the elements in flight and send them to remote? > They store the VirtQueueElement including the index and the sglists. > Can you explain *why* the index is not enough to reconstruct the state > on the destination? There may be bugs and you may need help from > virtio_blk_load, but that's okay. > > >>> You said that at the point where we save state, > >>> some entries are outstanding. It is too late to > >>> put head at that point. > >> > >> I don't want to put head on the source. I want to put it on the > >> destination, when the request is completed. Same as it is done now, > >> with bugfixes of course. Are there any problems doing so, except that > >> inuse will not be up-to-date (easily fixed)? > > > > You have an outstanding request that is behind last avail index. > > You do not want to complete it. You migrate. There is no > > way for remote to understand that the request is outstanding. > > The savevm callbacks know which request is outstanding and pass the > information to the destination. See virtio_blk_save and virtio_blk_load. > > What is not clear, and you haven't explained, is how you get to a bug in > the handling of the avail ring. What's wrong with this explanation: > > A 1 > A 2 > U 2 > A 2 > U 2 > A 2 > U 2 > A 2 <--- > U 2 > > where before the point marked with the arrow, the avail ring is > > 1 2 2 2 > > vring_avail_idx(vq) == 3 > last_avail_idx == 3 > > and after the point marked with the arrow, the avail ring is > > 2 2 2 2 > vring_avail_idx(vq) == 4 > last_avail_idx == 3 > > ?!? You need to retry A1 on remote. How do you do that? There's no way to find out it has not been completed from the ring itself. > >>>> It's not common, but you cannot block migration because you have an I/O > >>>> error. Solving the error may involve migrating the guests away from > >>>> that host. > >>> > >>> No, you should complete with error. > >> > >> Knowing that the request will fail, the admin will not be able to do > >> migration, even if that will solve the error transparently. > > > > You are saying there's no way to complete all requests? > > With an error, yes. Transparently after fixing the error (which may > involve migration), no. > > Paolo