[Qemu-devel] Combining -loadvm and -snapshot

[Drew DeVault <sir@cmpwn.com>]

I recently ran into an issue where I found I couldn't combine the
-loadvm and -snapshot flags, nor any conceivable combination of
alternate approaches like loadvm via the monitor. Independently, both
options work as expected, but together I get this error:

qemu-system-x86_64: Device 'virtio0' does not have the requested snapshot 'base'

The goal here is to resume the VM state from a snapshot, but to prevent
the guest from persisting writes to the underlying qcow2.

I started digging into the code to understand this problem more, and I
was pretty deep in the weeds when I realized what the underlying problem
probably was and the kind of refactoring necessary to fix it - so I'm
here to touch base before moving any further.

I believe this happens because -snapshot creates a temporary qcow2
overlaid on top of the disk you're using, and this overlay does not have
any snapshots copied, nor does any of the snapshot reading code (e.g.
qcow2_snapshot_list or qcow2_snapshot_goto) iterate over backing disks
to load their snapshots.

At first I was going to adjust the qcow2 snapshot loading code (those
two functions in particular) to read through their backends, but I'm a
little unfamiliar with this code and the refactoring is not minor so I
would like to get feedback from some of the wiser folks on this mailing
list before I sink too much time into this.

Thoughts?

--
Drew DeVault