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

[Kevin Wolf]

Am 03.07.2020 um 19:29 hat Denis V. Lunev geschrieben:
> 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 :(

Should we make this a job (may or may not be a block job) that just
happens to block the VM and return completion immediately with the
simple implementation we can have today? Then moving it later to a
truly async operation mode should become transparent to the QMP client.

Kevin