Re: [PATCH 2/5] KVM: svm: Disallow EFER.LMSLE on hardware that doesn't support it

[Sean Christopherson]

On Thu, Sep 15, 2022, Jim Mattson wrote:
> KVM has never properly virtualized EFER.LMSLE. When the "nested"
> module parameter is true, it allows an SVM guest to set EFER.LMSLE,
> and it passes the bit through in the VMCB, but the KVM emulator
> doesn't perform the required data segment limit checks in 64-bit mode.
> 
> With Zen3, AMD has dropped support for EFER.LMSLE. Hence, if a Zen3
> guest sets EFER.LMSLE, the next VMRUN will fail with "invalid VMCB."
> 
> When the host reports X86_FEATURE_NO_LMSLE, treat EFER.LMSLE as a
> reserved bit in the guest. Now, if a guest tries to set EFER.LMSLE on
> a host without support for EFER.LMSLE, the WRMSR will raise a #GP.
> 
> At the moment, the #GP may come as a surprise, but it's an improvement
> over the failed VMRUN. The #GP will be vindicated anon.
> 
> Fixes: eec4b140c924 ("KVM: SVM: Allow EFER.LMSLE to be set with nested svm")
> Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
> ---
>  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 f3813dbacb9f..7c4fd594166c 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -5012,7 +5012,9 @@ static __init int svm_hardware_setup(void)
>  
>  	if (nested) {
>  		printk(KERN_INFO "kvm: Nested Virtualization enabled\n");
> -		kvm_enable_efer_bits(EFER_SVME | EFER_LMSLE);
> +		kvm_enable_efer_bits(EFER_SVME);
> +		if (!boot_cpu_has(X86_FEATURE_NO_LMSLE))
> +			kvm_enable_efer_bits(EFER_LMSLE);

Since KVM doesn't correctly virtualize EFER.LMSLE, I wonder if we can get away with
dropping support entirely.  I.e. delete the reference to EFER_LMSLE and unconditionally
set F(NO_LMSLE) in KVM_GET_SUPPORTED_CPUID.