linux-kernel - [PATCH] KVM: x86: synthesize TSA CPUID bits via SCATTERED

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Message-ID: <20260208164233.30405-1-clopez@suse.de>
Date: Sun,  8 Feb 2026 17:42:33 +0100
From: Carlos López <clopez@...e.de>
To: seanjc@...gle.com,
	bp@...en8.de,
	kvm@...r.kernel.org
Cc: Carlos López <clopez@...e.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org (maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)),
	"H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org (open list:X86 ARCHITECTURE (32-BIT AND 64-BIT))
Subject: [PATCH] KVM: x86: synthesize TSA CPUID bits via SCATTERED_F()

KVM incorrectly synthesizes TSA_SQ_NO and TSA_L1_NO when running
on AMD Family 19h CPUs by using SYNTHESIZED_F(), which unconditionally
enables features for KVM-only CPUID leaves (as is the case with
CPUID_8000_0021_ECX), regardless of the kernel's synthesis logic in
tsa_init(). This is due to the following logic in kvm_cpu_cap_init():

    if (leaf < NCAPINTS)
        kvm_cpu_caps[leaf] &= kernel_cpu_caps[leaf];

This can cause an unexpected failure on Family 19h CPUs during SEV-SNP
guest setup, when userspace issues SNP_LAUNCH_UPDATE, as setting these
bits in the CPUID page on vulnerable CPUs is explicitly rejected by SNP
firmware.

Switch to SCATTERED_F(), so that the bits are only set if the features
have been force-set by the kernel in tsa_init(), or if they are reported
in the raw CPUID.

Fixes: 31272abd5974 ("KVM: SVM: Advertise TSA CPUID bits to guests")
Signed-off-by: Carlos López <clopez@...e.de>
---
 arch/x86/kvm/cpuid.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 88a5426674a1..819c176e02ff 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1230,8 +1230,8 @@ void kvm_set_cpu_caps(void)
 	);

 	kvm_cpu_cap_init(CPUID_8000_0021_ECX,
-		SYNTHESIZED_F(TSA_SQ_NO),
-		SYNTHESIZED_F(TSA_L1_NO),
+		SCATTERED_F(TSA_SQ_NO),
+		SCATTERED_F(TSA_L1_NO),
 	);

 	kvm_cpu_cap_init(CPUID_8000_0022_EAX,

base-commit: 0de4a0eec25b9171f2a2abb1a820e125e6797770
-- 
2.51.0