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(-) --=20 2.26.2
