Re: [PATCH v2 2/3] KVM: SEV: Add support for IBPB-on-Entry

[Sean Christopherson <seanjc@google.com>]

On Sat, Feb 28, 2026, Borislav Petkov wrote:
> Sean, ack for the KVM bits and me taking them thru tip?

Ya, should be fine for this to go through tip.

> On Tue, Feb 03, 2026 at 04:24:04PM -0600, Kim Phillips wrote:
> > AMD EPYC 5th generation and above processors support IBPB-on-Entry
> > for SNP guests.  By invoking an Indirect Branch Prediction Barrier
> > (IBPB) on VMRUN, old indirect branch predictions are prevented
> > from influencing indirect branches within the guest.
> > 
> > SNP guests may choose to enable IBPB-on-Entry by setting
> > SEV_FEATURES bit 21 (IbpbOnEntry).
> > 
> > Host support for IBPB on Entry is indicated by CPUID
> > Fn8000_001F[IbpbOnEntry], bit 31.
> > 
> > If supported, indicate support for IBPB on Entry in
> > sev_supported_vmsa_features bit 23 (IbpbOnEntry).
> > 
> > For more info, refer to page 615, Section 15.36.17 "Side-Channel
> > Protection", AMD64 Architecture Programmer's Manual Volume 2: System
> > Programming Part 2, Pub. 24593 Rev. 3.42 - March 2024 (see Link).
> > 
> > Link: https://bugzilla.kernel.org/attachment.cgi?id=306250
> > Signed-off-by: Kim Phillips <kim.phillips@amd.com>
> > Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
> > ---

...

> > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > index ea515cf41168..8a6d25db0c00 100644
> > --- a/arch/x86/kvm/svm/sev.c
> > +++ b/arch/x86/kvm/svm/sev.c
> > @@ -3165,8 +3165,15 @@ void __init sev_hardware_setup(void)
> >  	    cpu_feature_enabled(X86_FEATURE_NO_NESTED_DATA_BP))
> >  		sev_supported_vmsa_features |= SVM_SEV_FEAT_DEBUG_SWAP;
> >  
> > -	if (sev_snp_enabled && tsc_khz && cpu_feature_enabled(X86_FEATURE_SNP_SECURE_TSC))
> > +	if (!sev_snp_enabled)
> > +		return;
> > +	/* the following feature bit checks are SNP specific */
> > +
> > +	if (tsc_khz && cpu_feature_enabled(X86_FEATURE_SNP_SECURE_TSC))
> >  		sev_supported_vmsa_features |= SVM_SEV_FEAT_SECURE_TSC;
> > +
> > +	if (cpu_feature_enabled(X86_FEATURE_IBPB_ON_ENTRY))
> > +		sev_supported_vmsa_features |= SVM_SEV_FEAT_IBPB_ON_ENTRY;
> >  }

I think I'd prefer to nest the if-statement, e.g.

	if (sev_snp_enabled) {
		if (tsc_khz && cpu_feature_enabled(X86_FEATURE_SNP_SECURE_TSC))
			sev_supported_vmsa_features |= SVM_SEV_FEAT_SECURE_TSC;

		if (cpu_feature_enabled(X86_FEATURE_IBPB_ON_ENTRY))
			sev_supported_vmsa_features |= SVM_SEV_FEAT_IBPB_ON_ENTRY;
	}

I'm mildly concerned that'll we'll overlook the early return and unintentionally
bury common code in the SNP-section tail.

More importantly, this patch is buggy.  __sev_guest_init() needs to disallow
setting SVM_SEV_FEAT_IBPB_ON_ENTRY for non-SNP guests.

As a follow-up, I also think we should advertise SVM_SEV_FEAT_SNP_ACTIVE and
allow userspace to set the flag in kvm_sev_init.flags.  KVM still needs to set
the flag for backwards compatibility, but disallowing SVM_SEV_FEAT_SNP_ACTIVE
for an SNP guest is bizarre.

E.g. across 2 or 3 patches:

diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
index edde36097ddc..7db1bfce4cca 100644
--- a/arch/x86/include/asm/svm.h
+++ b/arch/x86/include/asm/svm.h
@@ -307,6 +307,10 @@ static_assert((X2AVIC_4K_MAX_PHYSICAL_ID & AVIC_PHYSICAL_MAX_INDEX_MASK) == X2AV
 #define SVM_SEV_FEAT_DEBUG_SWAP                                BIT(5)
 #define SVM_SEV_FEAT_SECURE_TSC                                BIT(9)
 
+#define SVM_SEV_FEAT_SNP_ONLY_MASK     (SVM_SEV_FEAT_SNP_ACTIVE | \
+                                        SVM_SEV_FEAT_SECURE_TSC | \
+                                        SVM_SEV_FEAT_IBPB_ON_ENTRY)
+
 #define VMCB_ALLOWED_SEV_FEATURES_VALID                        BIT_ULL(63)
 
 struct vmcb_seg {
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 41385573629e..b2fe0fa11f90 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -500,7 +500,7 @@ static int __sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp,
                return -EINVAL;
 
        if (!snp_active)
-               valid_vmsa_features &= ~SVM_SEV_FEAT_SECURE_TSC;
+               valid_vmsa_features &= ~SVM_SEV_FEAT_SNP_ONLY_MASK;
 
        if (data->vmsa_features & ~valid_vmsa_features)
                return -EINVAL;
@@ -3218,8 +3218,15 @@ void __init sev_hardware_setup(void)
            cpu_feature_enabled(X86_FEATURE_NO_NESTED_DATA_BP))
                sev_supported_vmsa_features |= SVM_SEV_FEAT_DEBUG_SWAP;
 
-       if (sev_snp_enabled && tsc_khz && cpu_feature_enabled(X86_FEATURE_SNP_SECURE_TSC))
-               sev_supported_vmsa_features |= SVM_SEV_FEAT_SECURE_TSC;
+       if (sev_snp_enabled) {
+               sev_supported_vmsa_features |= SVM_SEV_FEAT_SNP_ACTIVE;
+
+               if (tsc_khz && cpu_feature_enabled(X86_FEATURE_SNP_SECURE_TSC))
+                       sev_supported_vmsa_features |= SVM_SEV_FEAT_SECURE_TSC;
+
+               if (cpu_feature_enabled(X86_FEATURE_IBPB_ON_ENTRY))
+                       sev_supported_vmsa_features |= SVM_SEV_FEAT_IBPB_ON_ENTRY;
+       }
 }
 
 void sev_hardware_unsetup(void)