Re: swtpm persistent state snapshots by copying .permall file

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Sep 10, 2025 at 7:14 PM Stefan Berger <stefanb@linux.ibm.com> wrote:
> 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 for confirming!

Stefan