On Fri, Jul 03, 2020 at 08:29:08PM +0300, Denis V. Lunev wrote: > On 7/3/20 8:22 PM, Daniel P. Berrangé wrote: > > On Fri, Jul 03, 2020 at 08:15:44PM +0300, Denis V. Lunev wrote: > >> On 7/2/20 8:57 PM, Daniel P. Berrangé wrote: > >>> When QMP was first introduced some 10+ years ago now, the snapshot > >>> related commands (savevm/loadvm/delvm) were not converted. This was > >>> primarily because their implementation causes blocking of the thread > >>> running the monitor commands. This was (and still is) considered > >>> undesirable behaviour both in HMP and QMP. > >>> > >>> In theory someone was supposed to fix this flaw at some point in the > >>> past 10 years and bring them into the QMP world. Sadly, thus far it > >>> hasn't happened as people always had more important things to work > >>> on. Enterprise apps were much more interested in external snapshots > >>> than internal snapshots as they have many more features. > >>> > >>> Meanwhile users still want to use internal snapshots as there is > >>> a certainly simplicity in having everything self-contained in one > >>> image, even though it has limitations. Thus the apps that end up > >>> executing the savevm/loadvm/delvm via the "human-monitor-command" > >>> QMP command. > >>> > >>> > >>> IOW, the problematic blocking behaviour that was one of the reasons > >>> for not having savevm/loadvm/delvm in QMP is experienced by applications > >>> regardless. By not portting the commands to QMP due to one design flaw, > >>> we've forced apps and users to suffer from other design flaws of HMP ( > >>> bad error reporting, strong type checking of args, no introspection) for > >>> an additional 10 years. This feels rather sub-optimal :-( > >>> > >>> In practice users don't appear to care strongly about the fact that these > >>> commands block the VM while they run. I might have seen one bug report > >>> about it, but it certainly isn't something that comes up as a frequent > >>> topic except among us QEMU maintainers. Users do care about having > >>> access to the snapshot feature. > >>> > >>> Where I am seeing frequent complaints is wrt the use of OVMF combined > >>> with snapshots which has some serious pain points. This is getting worse > >>> as the push to ditch legacy BIOS in favour of UEFI gain momentum both > >>> across OS vendors and mgmt apps. Solving it requires new parameters to > >>> the commands, but doing this in HMP is super unappealing. > >>> > >>> > >>> > >>> After 10 years, I think it is time for us to be a little pragmatic about > >>> our handling of snapshots commands. My desire is that libvirt should never > >>> use "human-monitor-command" under any circumstances, because of the > >>> inherant flaws in HMP as a protocol for machine consumption. If there > >>> are flaws in QMP commands that's fine. If we fix them in future, we can > >>> deprecate the current QMP commands and remove them not too long after, > >>> without being locked in forever. > >>> > >>> > >>> Thus in this series I'm proposing a direct 1-1 mapping of the existing > >>> HMP commands for savevm/loadvm/delvm into QMP as a first step. This does > >>> not solve the blocking thread problem, but it does eliminate the error > >>> reporting, type checking and introspection problems inherant to HMP. > >>> We're winning on 3 out of the 4 long term problems. > >>> > >>> If someone can suggest a easy way to fix the thread blocking problem > >>> too, I'd be interested to hear it. If it involves a major refactoring > >>> then I think user are better served by unlocking what look like easy > >>> wins today. > >>> > >>> With a QMP variant, we reasonably deal with the problems related to OVMF: > >>> > >>> - The logic to pick which disk to store the vmstate in is not > >>> satsifactory. > >>> > >>> The first block driver state cannot be assumed to be the root disk > >>> image, it might be OVMF varstore and we don't want to store vmstate > >>> in there. > >>> > >>> - The logic to decide which disks must be snapshotted is hardwired > >>> to all disks which are writable > >>> > >>> Again with OVMF there might be a writable varstore, but this can be > >>> raw rather than qcow2 format, and thus unable to be snapshotted. > >>> While users might wish to snapshot their varstore, in some/many/most > >>> cases it is entirely uneccessary. Users are blocked from snapshotting > >>> their VM though due to this varstore. > >>> > >>> These are solved by adding two parameters to the commands. The first is > >>> a block device node name that identifies the image to store vmstate in, > >>> and the second is a list of node names to exclude from snapshots. > >>> > >>> In the block code I've only dealt with node names for block devices, as > >>> IIUC, this is all that libvirt should need in the -blockdev world it now > >>> lives in. IOW, I've made not attempt to cope with people wanting to use > >>> these QMP commands in combination with -drive args. > >>> > >>> I've done some minimal work in libvirt to start to make use of the new > >>> commands to validate their functionality, but this isn't finished yet. > >>> > >>> My ultimate goal is to make the GNOME Boxes maintainer happy again by > >>> having internal snapshots work with OVMF: > >>> > >>> > >>> https://gitlab.gnome.org/GNOME/gnome-boxes/-/commit/c486da262f6566326fbcb5e= > >>> f45c5f64048f16a6e > >>> > >>> Daniel P. Berrang=C3=A9 (6): > >>> migration: improve error reporting of block driver state name > >>> migration: introduce savevm, loadvm, delvm QMP commands > >>> block: add ability to filter out blockdevs during snapshot > >>> block: allow specifying name of block device for vmstate storage > >>> migration: support excluding block devs in QMP snapshot commands > >>> migration: support picking vmstate disk in QMP snapshot commands > >>> > >>> block/monitor/block-hmp-cmds.c | 4 +- > >>> block/snapshot.c | 68 +++++++++++++++++++------ > >>> include/block/snapshot.h | 21 +++++--- > >>> include/migration/snapshot.h | 10 +++- > >>> migration/savevm.c | 71 +++++++++++++++++++------- > >>> monitor/hmp-cmds.c | 20 ++------ > >>> qapi/migration.json | 91 ++++++++++++++++++++++++++++++++++ > >>> replay/replay-snapshot.c | 4 +- > >>> softmmu/vl.c | 2 +- > >>> 9 files changed, 228 insertions(+), 63 deletions(-) > >> I have tried to work in this interface in 2016. That time > >> we have struggled with the idea that this QMP interface should > >> be ready to work asynchronously. > >> > >> Write-protect userfaultfd was merged into vanilla Linux > >> thus it is time to async savevm interface, which will also > >> bring async loadvm and some rework for state storing. > >> > >> Thus I think that with the introduction of the QMP interface > >> we should at least run save VM not from the main > >> thread but from the background with the event at the end. > > spawning a thread in which to invoke save_snapshot() and load_snapshot() > > is easy enough. I'm not at all clear on what we need in the way of > > mutex locking though, to make those methods safe to run in a thread > > that isn't the main event loop. > > I am unsure that this is so easy. We need to be protected from other > operations > coming through QMP interface. Right now parallel operations are not allowed. > > > Even with savevm/loadvm being blocking, we could introduce a QMP event > > straight away, and document that users shouldn't assume the operation > > is complete until they see the event. That would let us make the commands > > non-blocking later with same documented semantics. > OK. Let us assume that you have added QMP savevm as proposed. It is > sync now. Sooner or later (I hope sooner) we will have to re-implement > this command with async version of the command, which will bring > again event etc and thus you will have to add compat layers to the > libvirt. > > I think that it would be cleaner to start with the interface suitable for > further (coming) features and not copy obsolete implementation. > Yes, unfortunately, this is much more complex :(
QMP-ifying the current design gives us a quick win by removing the dependancy on HMP, for negligible effort. It further lets us unlock a few extra useful features, again for negligible effort or maint burden, since we already have pretty much all the code already for HMP. Supporting internal snapshots is an explicit non-goal for Red Hat in terms of the products we're using QEMU with, as they all use external snapshots. Thus I'm very limited in time I can put into this feature myself. I am proposing this QMP-ification with my community hat on instead, as it looks like a quick win to help out various apps today, but it limits the amount of time I can justify spending on this. If someone wants to work on a perfect design that's great, but I don't want us to do nothing now, and continue to wait for a perfect solution that has shown no sign of arriving in 10 years. 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 :|