[Qemu-devel] Re: [PATCH 04/22] savevm: do_loadvm(): Always resume the VM

[Luiz Capitulino <lcapitulino@redhat.com>]

On Wed, 21 Apr 2010 15:28:16 +0200
Kevin Wolf <kwolf@redhat.com> wrote:

> Am 20.04.2010 23:09, schrieb Luiz Capitulino:
> > do_loadvm(), which implements the 'loadvm' Monitor command, pauses
> > the emulation to load the saved VM, however it will only resume
> > it if the loading succeeds.
> > 
> > In other words, if the user issues 'loadvm' and it fails, the
> > end result will be an error message and a paused VM.
> > 
> > This seems an undesirable side effect to me because, most of the
> > time, if a Monitor command fails the best thing we can do is to
> > leave the VM as it were before the command was executed.
> 
> I completely agree with Juan here, this is broken.

 Yeah, it's an RFC. I decided to send it as is because I needed feedback as
this series is a snowball.

> If you could leave the VM as it was before, just like you describe
> above, everything would be okay. But in fact you can't tell where the
> error occurred. You may still have the old state; or you may have loaded
> the snapshot on one disk, but not on the other one; or you may have
> loaded snapshots for all disks, but only half of the devices.
> 
> We must not run a machine in such an unknown state. I'd even go further
> and say that after a failed loadvm we must even prevent that a user uses
> the cont command to resume manually.

 Maybe 'info status' should have a specific status for this too.

 (Assuming we don't want to radically call exit(1)).

> > FIXME: This will try to run a potentially corrupted image, the
> >        solution is to split load_vmstate() in two and only keep
> >        the VM paused if qemu_loadvm_state() fails.
> 
> Your suggestion of having a prepare function that doesn't change any
> state looks fine to me. It could just check if the snapshot is there and
> contains VM state. This should cover all of the trivial cases where
> recovery is really as easy as resuming the old state.

 That's exactly what I want to do.

> Another problem that I see is that it's really hard to define what an
> error is. Current code print "Warning: ..." for all non-primary images
> and continues with loading the snapshot. I'm really unsure what the
> right behaviour would be if a snapshot doesn't exist on a secondary
> image (note that this is not the CD-ROM case, these drives are already
> excluded by bdrv_has_snapshot as they are read-only).

 Defining the right behavior and deciding what to expose have been
proven very difficult tasks in the QMP world.