Am 06.07.2020 um 18:03 hat Daniel P. Berrangé geschrieben: > 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. BerrangÃÆé wrote: > > > > > > On Fri, Jul 03, 2020 at 08:15:44PM +0300, Denis V. Lunev wrote: > > > > > >> On 7/2/20 8:57 PM, Daniel P. BerrangÃÆé 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. > > 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.
Yes, the job infrastructure is build on coroutines and we'd need to check that this is safe. But both loadvm and savevm call both vm_stop() and bdrv_drain_all_begin/end(), so not much should be going on in parallel. If this doesn't easily work out, there is still a simple solution for our sync implementation with an async interface: Just leave coroutine context immediately again by scheduling a BH that does the actual work. Kevin
