Re: [PATCH v6 00/10] migration: bring improved savevm/loadvm/delvm to QMP

[Daniel P . Berrangé]

ping: for anyone in the block or migration maintainers teams to
review this, since Markus has acked the QAPI design side of it.

On Thu, Oct 08, 2020 at 04:49:51PM +0100, Daniel P. Berrangé wrote:
>  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
>  v3: https://lists.gnu.org/archive/html/qemu-devel/2020-08/msg07076.html
>  v4: https://lists.gnu.org/archive/html/qemu-devel/2020-09/msg05221.html
>  v5: https://lists.gnu.org/archive/html/qemu-devel/2020-10/msg00587.html
> 
> This series aims to provide a better designed replacement for the
> savevm/loadvm/delvm HMP commands, which despite their flaws continue
> to be actively used in the QMP world via the HMP command passthrough
> facility.
> 
> The main problems addressed are:
> 
>  - 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.
> 
>  - The commands are synchronous blocking execution and returning
>    errors immediately.
> 
>    This is partially addressed by integrating with the job framework.
>    This forces the client to use the async commands to determine
>    the completion status or error message from the operations.
> 
> 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 v6:
> 
>  - Resolve many conflicts with recent replay changes
>  - Misc typos in QAPI
> 
> Changed in v5:
> 
>  - Fix prevention of tag overwriting
>  - Refactor and expand test suite coverage to validate
>    more negative scenarios
> 
> Changed in v4:
> 
>  - Make the device lists mandatory, dropping all support for
>    QEMU's built-in heuristics to select devices.
> 
>  - Improve some error reporting and I/O test coverage
> 
> 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 (10):
>   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
>   block: rename and alter bdrv_all_find_snapshot semantics
>   migration: control whether snapshots are ovewritten
>   migration: wire up support for snapshot device selection
>   migration: introduce a delete_snapshot wrapper
>   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               | 256 +++++++++++++++------
>  include/block/snapshot.h       |  23 +-
>  include/migration/snapshot.h   |  14 +-
>  migration/savevm.c             | 280 ++++++++++++++++++++---
>  monitor/hmp-cmds.c             |  12 +-
>  qapi/job.json                  |   9 +-
>  qapi/migration.json            | 121 ++++++++++
>  replay/replay-debugging.c      |  12 +-
>  replay/replay-snapshot.c       |   5 +-
>  softmmu/vl.c                   |   2 +-
>  tests/qemu-iotests/267.out     |  12 +-
>  tests/qemu-iotests/310         | 385 +++++++++++++++++++++++++++++++
>  tests/qemu-iotests/310.out     | 407 +++++++++++++++++++++++++++++++++
>  tests/qemu-iotests/common.qemu | 107 ++++++++-
>  tests/qemu-iotests/group       |   1 +
>  16 files changed, 1508 insertions(+), 145 deletions(-)
>  create mode 100755 tests/qemu-iotests/310
>  create mode 100644 tests/qemu-iotests/310.out
> 
> --=20
> 2.26.2
> 

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 :|