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

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Message-ID: <auftrtth2z3df4qc5vfm2cxv5b2tnhcu4eron7fbebalh6laxd@wgw2tamdw5ho>
Date: Wed, 1 Oct 2025 15:25:08 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Jim Mattson <jmattson@...gle.com>
Cc: 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>, 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
Subject: Re: [PATCH v2 2/2] KVM: SVM: Disallow EFER.LMSLE when not supported
 by hardware

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.

If anyone is as curious as I was, the bit seemingly started being set on
Milan. Rome (and supposedly older CPUs) support EFER.LMSLE.

> 
> 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
>