Re: [PATCH v10 04/50] x86/cpufeatures: Add SEV-SNP CPU feature

[Paolo Bonzini]

On 12/13/23 14:49, Borislav Petkov wrote:
> On Wed, Dec 13, 2023 at 02:40:24PM +0100, Paolo Bonzini wrote:
> > Why are they dead code?  X86_FEATURE_SEV_SNP is set automatically based on
> > CPUID, therefore patch 5 is a performance improvement on all processors that
> > support SEV-SNP.  This is independent of whether KVM can create SEV-SNP
> > guests or not.
>
> No, it is not. This CPUID bit means:
>
> "RMP table can be enabled to protect memory even from hypervisor."
>
> Without the SNP host patches, it is dead code.

-	if ((ia32_cap & ARCH_CAP_IBRS_ALL) || cpu_has(c, X86_FEATURE_AUTOIBRS)) {
+	if ((ia32_cap & ARCH_CAP_IBRS_ALL) ||
+	    (cpu_has(c, X86_FEATURE_AUTOIBRS) &&
+	     !cpu_feature_enabled(X86_FEATURE_SEV_SNP))) {

Surely we can agree that cpu_feature_enabled(X86_FEATURE_SEV_SNP) has nothing
to do with SEV-SNP host patches being present?  And that therefore retpolines
are preferred even without any SEV-SNP support in KVM?

And can we agree that "Acked-by" means "feel free and take it if you wish,
I don't care enough to merge it through my tree or provide a topic branch"?

I'm asking because I'm not sure if we agree on these two things, but they
really seem basic to me?

Paolo

> And regardless, arch/x86/kvm/ patches go through the kvm tree. The rest
> of arch/x86/ through the tip tree. We've been over this a bunch of times
> already.


> If you don't agree with this split, let's discuss it offlist with all
> tip and kvm maintainers, reach an agreement who picks up what and to put
> an end to this nonsense.
>
> Thx.