Re: [PATCH 0/6] migration: bring savevm/loadvm/delvm over to QMP

[Daniel P . Berrangé]

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.

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.

Regards,
Daniel
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