Re: [PATCH] i386/sev: Don't allow automatic fallback to legacy KVM_SEV*_INIT

[Paolo Bonzini <pbonzini@redhat.com>]

On Thu, Jul 4, 2024 at 11:39 AM Daniel P. Berrangé <berrange@redhat.com> wrote:
> > The debug_swap parameter simply could not be enabled in the old API
> > without breaking measurements. The new API *is the fix* to allow using
> > it (though QEMU doesn't have the option plumbed in yet). There is no
> > extensibility.
> >
> > Enabling debug_swap by default is also a thorny problem; it cannot be
> > enabled by default because not all CPUs support it, and also we'd have
> > the same problem that we cannot enable debug_swap on new machine types
> > without requiring a new kernel. Tying the default to the -cpu model
> > would work but it is confusing.
>
> Presumably we can tie it to '-cpu host' without much problem, and
> then just leave it as an opt-in feature flag for named CPU models.

'-cpu host' for SEV-SNP is also problematic, since CPUID is part of
the measurement. It's okay for starting guests in a quick and dirty
manner, but it cannot be used if measurement is in use.

It's weird to have "-cpu" provide the default for "-object", since
-object is created much earlier than CPUs. But then "-cpu" itself is
weird because it's a kind of "factory" for future objects. Maybe we
should redo the same exercise I did to streamline machine
initialization, this time focusing on -cpu/-machine/-accel, to
understand the various phases and where sev-{,snp-}guest
initialization fits.

> > I think it's reasonable if the fix is displayed right into the error
> > message. It's only needed for SEV-ES though, SEV can use the old and
> > new APIs interchangeably.
>
> FYI currently it is proposed to unconditionally force set legacy-vm-type=true
> in libvirt, so QEMU guests would *never* use the new ioctl, to fix what we
> consider to be a QEMU / kernel guest ABI regression.

Ok, so it's probably best to apply both this and your patch for now.
Later debug-swap can be enabled and will automatically disable
legacy-vm-type if the user left it to the default.

If you want to test this combo and send a pull request (either to me
or to Richard), that would help because I'm mostly away for a few
days.

Paolo