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

[Dr. David Alan Gilbert]

* James Bottomley (jejb@linux.ibm.com) wrote:
> 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.

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?)

Dave

> James
> 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK