linux-kernel - [PATCH 1/2] KVM: SEV: IBPB-on-Entry guest support

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Message-ID: <20260126224205.1442196-2-kim.phillips@amd.com>
Date: Mon, 26 Jan 2026 16:42:04 -0600
From: Kim Phillips <kim.phillips@....com>
To: <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<linux-coco@...ts.linux.dev>, <x86@...nel.org>
CC: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
	<pbonzini@...hat.com>, K Prateek Nayak <kprateek.nayak@....com>, "Nikunj A
 Dadhania" <nikunj@....com>, Tom Lendacky <thomas.lendacky@....com>, "Michael
 Roth" <michael.roth@....com>, Borislav Petkov <borislav.petkov@....com>,
	Borislav Petkov <bp@...en8.de>, Naveen Rao <naveen.rao@....com>, David Kaplan
	<david.kaplan@....com>, Kim Phillips <kim.phillips@....com>,
	<stable@...nel.org>
Subject: [PATCH 1/2] KVM: SEV: IBPB-on-Entry guest support

The SEV-SNP IBPB-on-Entry feature does not require a guest-side
implementation. The feature was added in Zen5 h/w, after the first
SNP Zen implementation, and thus was not accounted for when the
initial set of SNP features were added to the kernel.

In its abundant precaution, commit 8c29f0165405 ("x86/sev: Add SEV-SNP
guest feature negotiation support") included SEV_STATUS' IBPB-on-Entry
bit as a reserved bit, thereby masking guests from using the feature.

Unmask the bit, to allow guests to take advantage of the feature on
hypervisor kernel versions that support it: Amend the SEV_STATUS MSR
SNP_RESERVED_MASK to exclude bit 23 (IbpbOnEntry).

Fixes: 8c29f0165405 ("x86/sev: Add SEV-SNP guest feature negotiation support")
Cc: Nikunj A Dadhania <nikunj@....com>
Cc: Tom Lendacky <thomas.lendacky@....com>
CC: Borislav Petkov (AMD) <bp@...en8.de>
CC: Michael Roth <michael.roth@....com>
Cc: stable@...nel.org
Signed-off-by: Kim Phillips <kim.phillips@....com>
---
 arch/x86/boot/compressed/sev.c   | 1 +
 arch/x86/coco/sev/core.c         | 1 +
 arch/x86/include/asm/msr-index.h | 5 ++++-
 3 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index c8c1464b3a56..2b639703b8dd 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -188,6 +188,7 @@ bool sev_es_check_ghcb_fault(unsigned long address)
 				 MSR_AMD64_SNP_RESERVED_BIT13 |		\
 				 MSR_AMD64_SNP_RESERVED_BIT15 |		\
 				 MSR_AMD64_SNP_SECURE_AVIC |		\
+				 MSR_AMD64_SNP_RESERVED_BITS19_22 |	\
 				 MSR_AMD64_SNP_RESERVED_MASK)
 
 #ifdef CONFIG_AMD_SECURE_AVIC
diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 9ae3b11754e6..13f608117411 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -122,6 +122,7 @@ static const char * const sev_status_feat_names[] = {
 	[MSR_AMD64_SNP_VMSA_REG_PROT_BIT]	= "VMSARegProt",
 	[MSR_AMD64_SNP_SMT_PROT_BIT]		= "SMTProt",
 	[MSR_AMD64_SNP_SECURE_AVIC_BIT]		= "SecureAVIC",
+	[MSR_AMD64_SNP_IBPB_ON_ENTRY_BIT]	= "IBPBOnEntry",
 };
 
 /*
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 4d3566bb1a93..9016a6b00bc7 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -735,7 +735,10 @@
 #define MSR_AMD64_SNP_SMT_PROT		BIT_ULL(MSR_AMD64_SNP_SMT_PROT_BIT)
 #define MSR_AMD64_SNP_SECURE_AVIC_BIT	18
 #define MSR_AMD64_SNP_SECURE_AVIC	BIT_ULL(MSR_AMD64_SNP_SECURE_AVIC_BIT)
-#define MSR_AMD64_SNP_RESV_BIT		19
+#define MSR_AMD64_SNP_RESERVED_BITS19_22 GENMASK_ULL(22, 19)
+#define MSR_AMD64_SNP_IBPB_ON_ENTRY_BIT	23
+#define MSR_AMD64_SNP_IBPB_ON_ENTRY	BIT_ULL(MSR_AMD64_SNP_IBPB_ON_ENTRY_BIT)
+#define MSR_AMD64_SNP_RESV_BIT		24
 #define MSR_AMD64_SNP_RESERVED_MASK	GENMASK_ULL(63, MSR_AMD64_SNP_RESV_BIT)
 #define MSR_AMD64_SAVIC_CONTROL		0xc0010138
 #define MSR_AMD64_SAVIC_EN_BIT		0
-- 
2.43.0