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

Jim Mattson
2025-10-01T00:15:40Z

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@google.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