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

Yosry Ahmed
2025-10-01T15:18:50Z

On Tue, Sep 30, 2025 at 05:14:08PM -0700, Jim Mattson wrote:
> 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>

Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>

> ---
> 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
>