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[PATCH v3 0/7] migration: bring improved savevm/loadvm/delvm to QMP
From: |
Daniel P . Berrangé |
Subject: |
[PATCH v3 0/7] migration: bring improved savevm/loadvm/delvm to QMP |
Date: |
Thu, 27 Aug 2020 12:15:59 +0100 |
v1: https://lists.gnu.org/archive/html/qemu-devel/2020-07/msg00866.html
v2: https://lists.gnu.org/archive/html/qemu-devel/2020-07/msg07523.html
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.
Thus in this series I'm proposing a fairly direct 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 put in a place a
design using the jobs framework which can facilitate solving it later.
It does also solve the error reporting, type checking and introspection
problems inherant to HMP. So we're winning on 3 out of the 4 problems,
and pushed apps to a QMP design that will let us solve the last
remaining problem.
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 include for the snapshots.
If the list of nodes isn't given, it falls back to the historical
behaviour of using all disks matching some undocumented criteria.
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, as libvirt will
never use -drive with a QEMU new enough to have these new commands.
The main limitations of this current impl
- The snapshot process runs serialized in the main thread. ie QEMU
guest execution is blocked for the duration. The job framework
lets us fix this in future without changing the QMP semantics
exposed to the apps.
- Most vmstate loading errors just go to stderr, as they are not
using Error **errp reporting. Thus the job framework just
reports a fairly generic message
"Error -22 while loading VM state"
Again this can be fixed later without changing the QMP semantics
exposed to apps.
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
Changed in v3:
- Schedule a bottom half to escape from coroutine context in
the jobs. This is needed because the locking in the snapshot
code goes horribly wrong when run from a background coroutine
instead of the main event thread.
- Re-factor way we iterate over devices, so that we correctly
report non-existant devices passed by the user over QMP.
- Add QAPI docs notes about limitations wrt vmstate error
reporting (it all goes to stderr not an Error **errp)
so QMP only gets a fairly generic error message currently.
- Add I/O test to validate many usage scenarios / errors
- Add I/O test helpers to handle QMP events with a deterministic
ordering
- Ensure 'delete-snapshot' reports an error if requesting
delete from devices that don't support snapshot, instead of
silently succeeding with no erro.
Changed in v2:
- Use new command names "snapshot-{load,save,delete}" to make it
clear that these are different from the "savevm|loadvm|delvm"
as they use the Job framework
- Use an include list for block devs, not an exclude list
Daniel P. Berrang=C3=A9 (7):
migration: improve error reporting of block driver state name
block: push error reporting into bdrv_all_*_snapshot functions
migration: stop returning errno from load_snapshot()
block: add ability to specify list of blockdevs during snapshot
block: allow specifying name of block device for vmstate storage
iotests: add support for capturing and matching QMP events
migration: introduce snapshot-{save,load,delete} QMP commands
block/monitor/block-hmp-cmds.c | 7 +-
block/snapshot.c | 233 ++++++++++++++-------
include/block/snapshot.h | 19 +-
include/migration/snapshot.h | 10 +-
migration/savevm.c | 258 +++++++++++++++++++----
monitor/hmp-cmds.c | 11 +-
qapi/job.json | 9 +-
qapi/migration.json | 135 ++++++++++++
replay/replay-snapshot.c | 4 +-
softmmu/vl.c | 2 +-
tests/qemu-iotests/267.out | 14 +-
tests/qemu-iotests/310 | 255 +++++++++++++++++++++++
tests/qemu-iotests/310.out | 369 +++++++++++++++++++++++++++++++++
tests/qemu-iotests/common.qemu | 107 +++++++++-
tests/qemu-iotests/group | 1 +
15 files changed, 1289 insertions(+), 145 deletions(-)
create mode 100755 tests/qemu-iotests/310
create mode 100644 tests/qemu-iotests/310.out
--=20
2.26.2
- [PATCH v3 0/7] migration: bring improved savevm/loadvm/delvm to QMP,
Daniel P . Berrangé <=
- [PATCH v3 1/7] migration: improve error reporting of block driver state name, Daniel P . Berrangé, 2020/08/27
- [PATCH v3 2/7] block: push error reporting into bdrv_all_*_snapshot functions, Daniel P . Berrangé, 2020/08/27
- [PATCH v3 3/7] migration: stop returning errno from load_snapshot(), Daniel P . Berrangé, 2020/08/27
- [PATCH v3 4/7] block: add ability to specify list of blockdevs during snapshot, Daniel P . Berrangé, 2020/08/27
- [PATCH v3 5/7] block: allow specifying name of block device for vmstate storage, Daniel P . Berrangé, 2020/08/27
- [PATCH v3 6/7] iotests: add support for capturing and matching QMP events, Daniel P . Berrangé, 2020/08/27
- [PATCH v3 7/7] migration: introduce snapshot-{save, load, delete} QMP commands, Daniel P . Berrangé, 2020/08/27