[adding Daniel to CC]
On Tue, Aug 05, 2025 at 05:17:14AM -0700, Andrea Bolognani wrote:
> On Tue, Aug 05, 2025 at 12:56:56PM +0200, Gerd Hoffmann wrote:
> > > Assuming that
> > >
> > > * the need to use -bios for SEV-SNP is intended;
> >
> > Yes. SEV-SNP (and TDX too) are by design incompatible with pflash
> > emulation. Both do not allow the host change guest memory layout
> > after launch, and pflash needs to do that to switch between reading
> > mode and programming mode.
>
> Thanks for providing the additional insight.
>
> > > * pflash still needs to be used for SEV (-ES?);
> >
> > You can use pflash with SEV + SEV-ES. It makes sense to do that if
> > you want use a persistent variable store in pflash. Otherwise it
> > doesn't make much of a difference whenever you use -bios or read-only
> > pflash for the firmware.
>
> The current descriptor uses mode=stateless so there is not going to
> be a persistent variable store.
>
> > > then I think that we need to have the edk2 package ship two separate
> > > descriptors pointing to the same file, one containing
> > >
> > > {
> > > "mapping": {
> > > "device": "flash",
> > > "mode": "stateless",
> > > "executable": {
> > > "filename": "/usr/share/edk2/ovmf/OVMF.amdsev.fd",
> > > "format": "raw"
> > > }
> > > },
> > > "features": [
> > > "amd-sev",
> > > "amd-sev-es"
> > > ]
> > > }
> > >
> > > for SEV(-ES) and one containing
> > >
> > > {
> > > "mapping": {
> > > "device": "memory",
> > > "filename": "/usr/share/edk2/ovmf/OVMF.amdsev.fd"
> > > },
> > > "features": [
> > > "amd-sev-snp"
> > > ]
> > > }
> > >
> > > for SEV-SNP.
> >
> > That should work. Using device=memory for all three amd-sev* variants
> > should work too I think.
>
> Daniel suggested that elsewhere in the thread and of course it's an
> appealing proposition, as it would keep complexity down and unify
> handling across CVM use cases.
>
> However I wonder if changing things would break migration for
> existing SEV(-ES) guests. I think it would be fine since the current
> pflash-based configuration would be transmitted as part of the
> migration XML, so they will simply keep using that.
>
> If I'm right about the above, then I agree that we should just switch
> the existing SEV descriptor to device=memory.
--
Andrea Bolognani / Red Hat / Virtualization
