Re: [PATCH 2/2] tpm: add backend for mssim

[James Bottomley]

On Mon, 2023-01-09 at 16:59 +0000, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrangé (berrange@redhat.com) wrote:
> > On Fri, Dec 16, 2022 at 08:32:44AM -0500, Stefan Berger wrote:
[...]
> > > I do see it because the *volatile state* cannot be extracted from
> > > this device. The state of the PCRs is going to be lost.
> > 
> > All the objections you're raising are related to the current
> > specifics of the implementation of the mssim remote server.
> > While valid, this is of no concern to QEMU when deciding whether
> > to require a migration blocker on the client side. This is 3rd
> > party remote service that should be considered a black box from
> > QEMU's POV. It is possible to write a remote server that supports
> > the mssim network protocol, and has the ability to serialize
> > its state. Whether such an impl exists today or not is separate.
> 
> We would normally want an example of a working implementation though
> wouldn't we?
> 
> So I think it's fair to at least want some documentation; if it can
> be documented and works, fine; if it doesn't work, then it needs a
> blocker.

It works under limited circumstances ... in fact similar circumstances
passthrough migration works under, which is also not documented.  The
external MSSIM TPM emulator has to be kept running to preserve the
state.  If you restart it, the migration will fail.

James