Re: swtpm persistent state snapshots by copying .permall file

[Stefan Berger <stefanb@linux.ibm.com>]

On 9/10/25 5:08 PM, Stefan Hajnoczi wrote:
> Hi Stefan,
> I am investigating QEMU devices with persistent state like swtpm for a
> specific snapshot use case. The VM is paused while disk images and other
> persistent state files are snapshotted. This creates a crash-consistent
> snapshot similar to booting after power failure on a real machine. No
> RAM or volatile device state is collected.
> 
> My concern is how to ensure the swtpm's persistent state is captured as
> consistently as possible, but I'm not very familiar with the code. I
> wanted to run the following by you:
> 
> - Using --tpmstate dir= will write the persistent state to a new
>    temporary file and then atomically replace the old .permall file using
>    rename(2).

This is correct for the directory backend. We also have the file backend 
where a single file is keeping all the different types of state.

> 
> - If the VM is paused and a copy of the .permall file is taken, then
>    this copy is consistent. It may not reflect any in-progress changes
>    being written into a new temporary file, but that doesn't matter from

Correct.

>    the snapshot point of view since the VM is paused and it hasn't seen
>    the completion of in-progress TPM operations.

> 
> - The .volatilestate and .savestate files do not need to be captured in
>    the snapshot since the goal is just to achieve crash consistency.

Yes, I think so since the .volatilestate state would be written only in 
case of VM suspend or migration and the .savestate only upon suspend-to-RAM.

> 
> Does this sound reasonable or have I missed something?

No, I think this sounds reasonable.

> 
> Thanks,
> Stefan