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 10:42 AM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> On Thu, Jul 04, 2024 at 08:51:05AM +0200, Paolo Bonzini wrote:
> > On Thu, Jul 4, 2024 at 2:01 AM Michael Roth <michael.roth@amd.com> wrote:
> > > Currently if the 'legacy-vm-type' property of the sev-guest object is
> > > left unset, QEMU will attempt to use the newer KVM_SEV_INIT2 kernel
> > > interface in conjunction with the newer KVM_X86_SEV_VM and
> > > KVM_X86_SEV_ES_VM KVM VM types.
> > >
> > > This can lead to measurement changes if, for instance, an SEV guest was
> > > created on a host that originally had an older kernel that didn't
> > > support KVM_SEV_INIT2, but is booted on the same host later on after the
> > > host kernel was upgraded.
> >
> > I think this is the right thing to do for SEV-ES. I agree that it's
> > bad to require a very new kernel (6.10 will be released only a month
> > before QEMU 9.1), on the other hand the KVM_SEV_ES_INIT API is broken
> > in several ways. As long as there is a way to go back to it, and it's
> > not changed by old machine types, not using it for SEV-ES is the
> > better choice for upstream.
>
> Broken how ?   I know there was the regression with the 'debug_swap'
> parameter, but was something that should just be fixed in the kernel,
> rather than breaking userspace. What else is a problem ?

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.

But I guess we can add support for debug_swap, disabled by default and
switch to the new API if debug_swap is enabled.

> I don't think its reasonable for QEMU to require a brand new kernel
> for new machine types, given SEV & SEV-ES have been deployed for
> many years already.

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.

Paolo