Re: [pve-devel] [PATCH v3 qemu-server] Fix ACPI-suspended VMs resuming after migration

[Fiona Ebner <f.ebner@proxmox.com>]

Am 10.10.23 um 16:19 schrieb Fiona Ebner:
>> Also, with that change we might have added a race for suspend-mode backups,
>> at least if VMs really can wake up without a QMP command (which I find likely).
>> I.e., between the time we checked and set vm_was_paused until we actually
>> suspend, because if the VM would wake up in between we might get inconsistent
>> stuff and skip things like fs-freeze.
> 
> That race is already there without the patch. QEMU does not transition
> from 'suspended' state to 'paused' state when QMP 'stop' is issued, i.e.
> what our 'qm suspend' or vm_suspend() actually does. So it doesn't
> matter if we call that during backup or not when the VM is already in
> 'suspended' state.
> 
> The window for the race is a bit larger though:
> now: VM wakes up between check if paused and 'backup' QMP
> before: VM wakes up after fsfreeze was skipped because guest agent was
> detected as not running
> 

What's written above actually applies to 'snapshot' mode backup, but not
to 'suspend' mode backup. 'suspend' mode backup is and was broken in
this regard already:

> root@pve8a1 ~ # vzdump 100 --storage pbs --mode suspend 
> INFO: starting new backup job: vzdump 100 --mode suspend --storage pbs
> INFO: Starting Backup of VM 100 (qemu)
> INFO: Backup started at 2023-10-11 09:25:53
> INFO: status = running
> INFO: backup mode: suspend
> INFO: ionice priority: 7
> INFO: VM Name: Copy-of-VM-apache
> INFO: include disk 'scsi0' 'rbd:vm-100-disk-0' 4618348134
> INFO: suspending guest
> INFO: snapshots found (not included into backup)
> INFO: creating Proxmox Backup Server archive 'vm/100/2023-10-11T07:25:53Z'
> INFO: skipping guest-agent 'fs-freeze', agent configured but not running?
> INFO: started backup task '923c95cf-6bb6-4476-825e-390c147a4e76'
> INFO: resuming VM again after 3 seconds
> INFO: scsi0: dirty-bitmap status: OK (8.0 MiB of 4.3 GiB dirty)
> INFO: using fast incremental mode (dirty-bitmap), 8.0 MiB dirty of 4.3 GiB total
> INFO: 100% (8.0 MiB of 8.0 MiB) in 1s, read: 8.0 MiB/s, write: 8.0 MiB/s
> INFO: backup was done incrementally, reused 4.29 GiB (99%)
> INFO: transferred 8.00 MiB in 1 seconds (8.0 MiB/s)
> INFO: adding notes to backup
> INFO: resume vm
> INFO: Finished Backup of VM 100 (00:00:06)
> INFO: Backup finished at 2023-10-11 09:25:59
> INFO: Backup job finished successfully

We never got the fsfreeze in suspend mode, because we do it after the
QMP 'stop' when the vCPUs are paused, so the guest agent won't be
running. The backup just ends up with whatever state the filesystem was
in when QMP 'stop' is issued.

Time to finally deprecate 'suspend' mode backup for VMs for good? It
really has just disadvantages compared to 'snapshot' mode and our docs
state "This mode is provided for compatibility reason" for a long time now.

> We could either:
> 
> 1. (ironically) disallow 'suspend' mode backup when in 'suspended' runstate.
> 2. resume and pause the VM upon 'suspend' mode backup, but is also
> surprising
> 3. make the race window smaller again by doing the qga_check_running()
> and fs-freeze if true, if VM was in 'suspended' runstate.
> 
> I don't see how to avoid the possibility of a wake-up during an
> inconvenient time except with one of the first two options.
>