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 -- |: 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 :|
