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

[Daniel P . Berrangé]

On Mon, Jul 06, 2020 at 07:10:16PM +0300, Denis V. Lunev wrote:
> On 7/6/20 7:03 PM, Daniel P. Berrangé wrote:
> > On Mon, Jul 06, 2020 at 05:50:11PM +0200, Kevin Wolf wrote:
> >> Am 06.07.2020 um 17:29 hat Daniel P. Berrangé geschrieben:
> >>> On Mon, Jul 06, 2020 at 05:27:01PM +0200, Kevin Wolf wrote:
> >>>> 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.
> >>> What would making it a job / block job need from a QMP design POV ?
> >> The actual QMP syntax for the command wouldn't look much different (I
> >> think just a new option 'job-id'), but the difference would be that it's
> >> not documented as performing the whole action, but just starting the
> >> job. The expectation would then be that it can be managed with the
> >> job-* commands and that it emits the job status events.
> >>
> >> This may sound complicated, but most of it is actually covered by the
> >> generic job infrastructure.
> >>
> >> The simplest job that we have is blockdev-create, which is implemented
> >> in block/create.c (99 lines including the license header). I think this
> >> would be a good model for our new case.
> This proposal looks perfect to me!
> 
> > The QMP design and internal API looks simple enough, but I'm wondering
> > what implications come with the job infra wrt locking/thread safety. In
> > particular I see the "job_start" command runs the impl in a coroutine.
> > I can't tell if that's going to cause any interactions wrto the current
> > loadvm/savevm impl and its assumptions about blocking execution while
> > running.
>
> Right now we don't care. This is API part. For the implentation part the
> code remains as-is. In this case we just adopt libvirt to the new
> approach while QEMU remains old. Converting QEMU to new iface
> is indeed separate (much more complex) task.

If we're exposing a "job-id" in the QAPI schema though, applications are
going to expect the normal "job-XXX" commands to be functional.

We don't want to have a "job-id" that can't be used right now, and then
magically starts working later, because there'll be no way for apps to
introspect whether "job-id" works or not.

So if we expose job-id it needs to do something IMHO, otherwise we should
not expose job-id at all, and simply add it to the command later  once it
does actally do something useful.

Regards,
Daniel
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