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

[Daniel P . Berrangé]

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 ?

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. 


With regards,
Daniel
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