* James Bottomley (jejb@xxxxxxxxxxxxx) wrote: > On Thu, 2021-08-19 at 09:22 +0100, Dr. David Alan Gilbert wrote: > > * Tobin Feldman-Fitzthum (tobin@xxxxxxxxxxxxx) wrote: > > > On 8/18/21 3:04 PM, Dr. David Alan Gilbert wrote: > > > > * Tobin Feldman-Fitzthum (tobin@xxxxxxxxxxxxx) 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@xxxxxxxxxx / Manchester, UK
