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Message-ID: <85o6qjqgvh.fsf@amd.com>
Date: Tue, 7 Oct 2025 08:02:26 +0000
From: Nikunj A Dadhania <nikunj@....com>
To: Jim Mattson <jmattson@...gle.com>, 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: Re: [PATCH v2 2/2] KVM: SVM: Disallow EFER.LMSLE when not supported
by hardware
Jim Mattson <jmattson@...gle.com> writes:
> 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>
Reviewed-by: Nikunj A Dadhania <nikunj@....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
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