linux-kernel - [PATCH v2 2/2] KVM: SVM: Disallow EFER.LMSLE when not supported by hardware

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Message-ID: <20251001001529.1119031-3-jmattson@google.com>
Date: Tue, 30 Sep 2025 17:14:08 -0700
From: Jim Mattson <jmattson@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Tom Lendacky <thomas.lendacky@....com>, 
	Jim Mattson <jmattson@...gle.com>, Perry Yuan <perry.yuan@....com>, 
	Sohil Mehta <sohil.mehta@...el.com>, "Xin Li (Intel)" <xin@...or.com>, 
	Joerg Roedel <joerg.roedel@....com>, Avi Kivity <avi@...hat.com>, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: [PATCH v2 2/2] KVM: SVM: Disallow EFER.LMSLE when not supported by hardware

Modern AMD CPUs do not support segment limit checks in 64-bit mode
(i.e. EFER.LMSLE must be zero). Do not allow a guest to set EFER.LMSLE
on a CPU that requires the bit to be zero.

For backwards compatibility, allow EFER.LMSLE to be set on CPUs that
support segment limit checks in 64-bit mode, even though KVM's
implementation of the feature is incomplete (e.g. KVM's emulator does
not enforce segment limits in 64-bit mode).

Fixes: eec4b140c924 ("KVM: SVM: Allow EFER.LMSLE to be set with nested svm")

Signed-off-by: Jim Mattson <jmattson@...gle.com>
---
 arch/x86/kvm/svm/svm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 1bfebe40854f..78d0fc85d0bd 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5351,7 +5351,9 @@ static __init int svm_hardware_setup(void)
 
 	if (nested) {
 		pr_info("Nested Virtualization enabled\n");
-		kvm_enable_efer_bits(EFER_SVME | EFER_LMSLE);
+		kvm_enable_efer_bits(EFER_SVME);
+		if (!boot_cpu_has(X86_FEATURE_EFER_LMSLE_MBZ))
+			kvm_enable_efer_bits(EFER_LMSLE);
 
 		r = nested_svm_init_msrpm_merge_offsets();
 		if (r)
-- 
2.51.0.618.g983fd99d29-goog