Re: [RFC PATCH 00/13] Add support for Mirror VM.

[James Bottomley]

On Thu, 2021-08-19 at 15:28 +0100, Dr. David Alan Gilbert wrote:
> * James Bottomley (jejb@linux.ibm.com) wrote:
> > On Thu, 2021-08-19 at 09:22 +0100, Dr. David Alan Gilbert wrote:
[...]
> > > I think it really does have to cope with migration to a new
> > > version of host.
> > 
> > Well, you're thinking of OVMF as belonging to the host because of
> > the way it is supplied, but think about the way it works in
> > practice now, forgetting about confidential computing: OVMF is RAM
> > resident in ordinary guests, so when you migrate them, the whole of
> > OVMF (or at least what's left at runtime) goes with the migration,
> > thus it's not possible to change the guest OVMF by migration.  The
> > above is really just an extension of that principle, the only
> > difference for confidential computing being you have to have an
> > image of the current OVMF ROM in the target to seed migration.
> > 
> > Technically, the problem is we can't overwrite running code and
> > once the guest is re-sited to the target, the OVMF there has to
> > match exactly what was on the source for the RT to still
> > function.   Once the migration has run, the OVMF on the target must
> > be identical to what was on the source (including internally
> > allocated OVMF memory), and if we can't copy the MH code, we have
> > to rely on the target image providing this identical code and we
> > copy the rest.
> 
> I'm OK with the OVMF now being part of the guest image, and having to
> exist on both; it's a bit delicate though unless we have a way to
> check it (is there an attest of the destination happening here?)

There will be in the final version.  The attestations of the source and
target, being the hash of the OVMF (with the registers in the -ES
case), should be the same (modulo any firmware updates to the PSP,
whose firmware version is also hashed) to guarantee the OVMF is the
same on both sides.  We'll definitely take an action to get QEMU to
verify this ... made a lot easier now we have signed attestations ...

James