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. > > Regards, > Daniel 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. Den
