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client-ip=165.204.84.17; helo=satlexmb07.amd.com; pr=C Received: from satlexmb07.amd.com (165.204.84.17) by CH1PEPF0000AD80.mail.protection.outlook.com (10.167.244.90) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9564.3 via Frontend Transport; Tue, 27 Jan 2026 06:38:37 +0000 Received: from satlexmb10.amd.com (10.181.42.219) by satlexmb07.amd.com (10.181.42.216) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.17; Tue, 27 Jan 2026 00:38:37 -0600 Received: from satlexmb08.amd.com (10.181.42.217) by satlexmb10.amd.com (10.181.42.219) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.17; Tue, 27 Jan 2026 00:38:36 -0600 Received: from [10.136.47.140] (10.180.168.240) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server id 15.2.2562.17 via Frontend Transport; Tue, 27 Jan 2026 00:38:33 -0600 Message-ID: <4dea11f9-6034-489b-acaf-9a150818d1a1@amd.com> Date: Tue, 27 Jan 2026 12:08:27 +0530 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] KVM: SEV: Add support for IBPB-on-Entry To: Kim Phillips , , , , CC: Sean Christopherson , Paolo Bonzini , K Prateek Nayak , Tom Lendacky , Michael Roth , "Borislav Petkov" , Borislav Petkov , Naveen Rao , David Kaplan References: <20260126224205.1442196-1-kim.phillips@amd.com> <20260126224205.1442196-3-kim.phillips@amd.com> Content-Language: en-US From: "Nikunj A. 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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 > --- > arch/x86/include/asm/cpufeatures.h | 1 + > arch/x86/include/asm/svm.h | 1 + > arch/x86/kvm/svm/sev.c | 9 ++++++++- > 3 files changed, 10 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h > index c01fdde465de..3ce5dff36f78 100644 > --- a/arch/x86/include/asm/cpufeatures.h > +++ b/arch/x86/include/asm/cpufeatures.h > @@ -459,6 +459,7 @@ > #define X86_FEATURE_ALLOWED_SEV_FEATURES (19*32+27) /* Allowed SEV Features */ > #define X86_FEATURE_SVSM (19*32+28) /* "svsm" SVSM present */ > #define X86_FEATURE_HV_INUSE_WR_ALLOWED (19*32+30) /* Allow Write to in-use hypervisor-owned pages */ > +#define X86_FEATURE_IBPB_ON_ENTRY (19*32+31) /* SEV-SNP IBPB on VM Entry */ > > /* AMD-defined Extended Feature 2 EAX, CPUID level 0x80000021 (EAX), word 20 */ > #define X86_FEATURE_NO_NESTED_DATA_BP (20*32+ 0) /* No Nested Data Breakpoints */ > diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h > index edde36097ddc..eebc65ec948f 100644 > --- a/arch/x86/include/asm/svm.h > +++ b/arch/x86/include/asm/svm.h > @@ -306,6 +306,7 @@ static_assert((X2AVIC_4K_MAX_PHYSICAL_ID & AVIC_PHYSICAL_MAX_INDEX_MASK) == X2AV > #define SVM_SEV_FEAT_ALTERNATE_INJECTION BIT(4) > #define SVM_SEV_FEAT_DEBUG_SWAP BIT(5) > #define SVM_SEV_FEAT_SECURE_TSC BIT(9) > +#define SVM_SEV_FEAT_IBPB_ON_ENTRY BIT(21) > > #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 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 */ > + The early return seems to split up the SNP features unnecessarily. Keeping everything under `if (sev_snp_enabled)` is cleaner IMO - it's clear that these features belong together. Plus, when someone adds the next SNP feature, they won't have to think about whether it goes before or after the return. The comment about "SNP specific" features becomes redundant as well. > + 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; Regards, Nikunj