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CPU hardware tracks each > guest's BTB entries and can flush the BTB if it has been determined > to be contaminated with any prediction information originating outside > the particular guest's context. > > To mitigate possible performance penalties incurred by these flushes, > it is recommended that the hypervisor run with SPEC_CTRL[IBRS] set. > Note that using Automatic IBRS is not an equivalent option here, since > it behaves differently when SEV-SNP is active. See commit acaa4b5c4c85 > ("x86/speculation: Do not enable Automatic IBRS if SEV-SNP is enabled") > for more details. > > Indicate support for BTB Isolation in sev_supported_vmsa_features, > bit 7. > > SNP-active guests can enable (BTB) Isolation through SEV_Status > bit 9 (SNPBTBIsolation). > > 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 > --- > v2: No changes > v1: https://lore.kernel.org/kvm/20260224180157.725159-4-kim.phillips@amd.com/ > > arch/x86/include/asm/svm.h | 1 + > arch/x86/kvm/svm/sev.c | 3 +++ > 2 files changed, 4 insertions(+) > > diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h > index edde36097ddc..2038461c1316 100644 > --- a/arch/x86/include/asm/svm.h > +++ b/arch/x86/include/asm/svm.h > @@ -305,6 +305,7 @@ static_assert((X2AVIC_4K_MAX_PHYSICAL_ID & AVIC_PHYSICAL_MAX_INDEX_MASK) == X2AV > #define SVM_SEV_FEAT_RESTRICTED_INJECTION BIT(3) > #define SVM_SEV_FEAT_ALTERNATE_INJECTION BIT(4) > #define SVM_SEV_FEAT_DEBUG_SWAP BIT(5) > +#define SVM_SEV_FEAT_BTB_ISOLATION BIT(7) > #define SVM_SEV_FEAT_SECURE_TSC BIT(9) > > #define VMCB_ALLOWED_SEV_FEATURES_VALID BIT_ULL(63) > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c > index 3f9c1aa39a0a..ac29cf47dd08 100644 > --- a/arch/x86/kvm/svm/sev.c > +++ b/arch/x86/kvm/svm/sev.c > @@ -3167,6 +3167,9 @@ void __init sev_hardware_setup(void) > > 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_BTB_ISOLATION; This would also need to update the SVM_SEV_FEAT_SNP_ONLY_MASK that Sean suggested/created in the IBPB-On-Entry series. Thanks, Tom > } > > void sev_hardware_unsetup(void)