On Tue, Dec 14, 2010 at 08:41:55AM -0700, Alex Williamson wrote: > On Tue, 2010-12-14 at 14:32 +0200, Michael S. Tsirkin wrote: > > On Mon, Dec 13, 2010 at 10:00:48PM -0700, Alex Williamson wrote: > > > On Tue, 2010-12-14 at 06:43 +0200, Michael S. Tsirkin wrote: > > > > On Mon, Dec 13, 2010 at 12:15:08PM -0700, Alex Williamson wrote: > > > > > On Mon, 2010-12-13 at 21:06 +0200, Michael S. Tsirkin wrote: > > > > > > On Mon, Dec 13, 2010 at 11:59:16AM -0700, Alex Williamson wrote: > > > > > > > On Mon, 2010-12-13 at 20:54 +0200, Michael S. Tsirkin wrote: > > > > > > > > On Mon, Dec 13, 2010 at 11:00:44AM -0700, Alex Williamson wrote: > > > > > > > > > On Mon, 2010-12-13 at 19:50 +0200, Michael S. Tsirkin wrote: > > > > > > > > > > On Mon, Dec 13, 2010 at 10:43:22AM -0700, Alex Williamson > > > > > > > > > > wrote: > > > > > > > > > > > So, unfortunately, I stand by my original patch. > > > > > > > > > > > > > > > > > > > > What about the one that put -1 in saved index for a > > > > > > > > > > hotplugged device? > > > > > > > > > > > > > > > > > > There are still examples that don't work even without hotplug > > > > > > > > > (example 2 > > > > > > > > > and example 3 after the reboot). That hack limits the > > > > > > > > > damage, but still > > > > > > > > > leaves a latent bug for reboot and doesn't address the > > > > > > > > > non-hotplug > > > > > > > > > scenarios. So, I don't think it's worthwhile to pursue, and > > > > > > > > > we > > > > > > > > > shouldn't pretend we can use it to avoid bumping the > > > > > > > > > version_id. > > > > > > > > > Thanks, > > > > > > > > > > > > > > > > > > Alex > > > > > > > > > > > > > > > > I guess when we bump it we tell users: migration is completely > > > > > > > > borken to the old version, don't even try it. > > > > > > > > > > > > > > > > Is there a way for libvirt to discover such incompatibilities > > > > > > > > and avoid the migration? > > > > > > > > > > > > > > I don't know if libvirt has a way to query this in advance. If a > > > > > > > migration is attempted, the target will report: > > > > > > > > > > > > > > savevm: unsupported version 5 for '0000:00:03.0/rtl8139' v4 > > > > > > > > > > > > > > And the source will continue running. We waste plenty of bits > > > > > > > getting > > > > > > > to that point, > > > > > > > > > > > > Yes, this happens after all of memory has been migrated. > > > > > > > > > > Better late than never :^\ > > > > > > > > One other question: can we do the same by creating a new (empty) > > > > section? As was discussed in the past this is easier for > > > > downstreams to cherry-pick. > > > > > > The only way I can think to do that would be to have a subsection that > > > is always included, but saves no data. That would force a failure on > > > new->old migration, but I don't think it really matches the intended > > > purpose of subsections and feels like it's adding cruft for no gain. > > > Maybe I'm missing something. Juan, is there any advantage to trapping > > > this in a subsection? Thanks, > > > > > > Alex > > > > Maybe in this particular case the advantage is minimal. > > But it seems easier to stick to a rule of no more version > > bumps than argue about each case. > > Do we have such a rule?
I think it's easier to have a hard rule than start arguing whether each change is a subsection or not for each patch. What is the downside? > If we have a subsection who's needed function > is return 1, I think that's a good indication that it's not appropriate > for a subsection and the end result is equivalent to bumping the main > driver vmstate version. So if you feel strongly about it, go ahead, I'm just concerned we'll now need to argue version or subsection for each migration-related change. > It's convoluted to try to hide a one-way upgrade in a subsection. It may be convoluted but I think it works better with downstreams. When we'll bump the version again in the future, it should be possible for a downstream to pick up just the next change, and skip this one. But that's a general comment. In this case, I expect all downstream to be able to pick this bugfix with no real downsides. > Thanks, > > Alex