On Fri, Jul 03, 2020 at 05:10:12PM +0100, Dr. David Alan Gilbert wrote: > * Daniel P. Berrangé (berra...@redhat.com) wrote: > > On Fri, Jul 03, 2020 at 04:49:33PM +0100, Dr. David Alan Gilbert wrote: > > > * Daniel P. Berrangé (berra...@redhat.com) wrote: > > > > On Thu, Jul 02, 2020 at 01:12:52PM -0500, Eric Blake wrote: > > > > > On 7/2/20 12:57 PM, Daniel P. Berrangé wrote: > > > > > > savevm, loadvm and delvm are some of the few commands that have > > > > > > never > > > > > > been converted to use QMP. The primary reason for this lack of > > > > > > conversion is that they block execution of the thread for as long as > > > > > > they run. > > > > > > > > > > > > Despite this downside, however, libvirt and applications using > > > > > > libvirt > > > > > > has used these commands for as long as QMP has existed, via the > > > > > > "human-monitor-command" passthrough command. IOW, while it is > > > > > > clearly > > > > > > desirable to be able to fix the blocking problem, this is not an > > > > > > immediate obstacle to real world usage. > > > > > > > > > > > > Meanwhile there is a need for other features which involve adding > > > > > > new > > > > > > parameters to the commands. This is possible with HMP passthrough, > > > > > > but > > > > > > it provides no reliable way for apps to introspect features, so > > > > > > using > > > > > > QAPI modelling is highly desirable. > > > > > > > > > > > > This patch thus introduces trival savevm, loadvm, delvm commands > > > > > > > > > > trivial > > > > > > > > > > > to QMP that are functionally identical to the HMP counterpart, > > > > > > including > > > > > > the blocking problem. > > > > > > > > > > Should we name them 'x-savevm', 'x-loadvm', 'x-delvm' to give > > > > > ourselves room > > > > > to change them when we DO solve the blocking issue? Or will the > > > > > solution of > > > > > the blocking issue introduce new QMP commands, at which point we can > > > > > add QMP > > > > > deprecation markers on these commands to eventually retire them? > > > > > > > > I was in two minds about this, so I'm open to arguments either way. > > > > > > > > The primary goal is for libvirt to consume the APIs as soon as possible, > > > > and generally libvirt doesn't want todo this is they are declared > > > > experimental > > > > via a "x-" prefix. So that pushes me away from "x-". > > > > > > > > If we don't have an "x-" prefix and want to make changes, we can add > > > > extra > > > > parameters to trigger new behaviour in backwards compatible manner. Or > > > > we can > > > > simply deprecate these commands, deleting them 2 releases later, while > > > > adding > > > > completely new commands. > > > > > > > > If we think the prposed design will definitely need incompatible > > > > changes in > > > > a very short time frame though, that would push towards "x-". > > > > > > > > So IMHO the right answer largely depends on whether there is a credible > > > > strategy to implement the ideal non-blocking solution in a reasonable > > > > amount > > > > of time. I can't justify spending much time on this myself, but I'm > > > > willing > > > > to consider & try proposals for solving the blocking problem if they're > > > > not > > > > too complex / invasive. > > > > > > Remind me, what was the problem with just making a block: migration > > > channel, and then we can migrate to it? > > > > migration only does vmstate, not disks. The current blockdev commands > > are all related to external snapshots, nothing for internal snapshots > > AFAIK. So we still need commands to load/save internal snapshots of > > the disk data in the qcow2 files. > > > > So we could look at loadvm/savevm conceptually as a syntax sugar above > > a migration transport that targets disk images, and blockdev QMP command > > that can do internal snapshots. Neither of these exist though and feel > > like a significantly larger amount of work than using existing functionality > > that is currently working. > > I think that's what we should aim for; adding this wrapper isn't gaining > that much without moving a bit towards that; so I would stick with the > x- for now.
The question is how much work that approach will be and whether anyone can realistically commit to doing that ? It looks like a much larger piece of work in both QEMU and libvirt side to do that. I don't want to see us stuck with a x-savevm for years because no one has resource to work on the perfect solution. If we did get a perfect solution at a point in future, we can still deprecate and then remove any "savevm" command we add to QMP. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
