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

[James Bottomley <jejb@linux.ibm.com>]

On Thu, 2021-08-19 at 09:22 +0100, Dr. David Alan Gilbert wrote:
> * Tobin Feldman-Fitzthum (tobin@linux.ibm.com) wrote:
> > On 8/18/21 3:04 PM, Dr. David Alan Gilbert wrote:
> > > * Tobin Feldman-Fitzthum (tobin@linux.ibm.com) wrote:
> > > > On 8/17/21 6:04 PM, Steve Rutherford wrote:
> > > > > Ahh, It sounds like you are looking into sidestepping the
> > > > > existing AMD-SP flows for migration. I assume the idea is to
> > > > > spin up a VM on the target side, and have the two VMs attest
> > > > > to each other. How do the two sides know if the other is
> > > > > legitimate? I take it that the source is directing the LAUNCH
> > > > > flows?
> > > >  
> > > > Yeah we don't use PSP migration flows at all. We don't need to
> > > > send the MH code from the source to the target because the MH
> > > > lives in firmware, which is common between the two.
> > >  
> > > Are you relying on the target firmware to be *identical* or
> > > purely for it to be *compatible* ?  It's normal for a migration
> > > to be the result of wanting to do an upgrade; and that means the
> > > destination build of OVMF might be newer (or older, or ...).
> > > 
> > > Dave
> > 
> > This is a good point. The migration handler on the source and
> > target must have the same memory footprint or bad things will
> > happen. Using the same firmware on the source and target is an easy
> > way to guarantee this. Since the MH in OVMF is not a contiguous
> > region of memory, but a group of functions scattered around OVMF,
> > it is a bit difficult to guarantee that the memory footprint is the
> > same if the build is different.
> 
> Can you explain what the 'memory footprint' consists of? Can't it
> just be the whole of the OVMF rom space if you have no way of nudging
> the MH into it's own chunk?

It might be possible depending on how we link it. At the moment it's
using the core OVMF libraries, but it is possible to retool the OVMF
build to copy those libraries into the MH DXE.

> 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.

James