Re: [PATCH] KVM: vmx, svm: always run with EFER.NXE=1 when shadow paging is active

[Joerg Roedel]

Hi Paolo,

On Sun, Oct 27, 2019 at 04:23:23PM +0100, Paolo Bonzini wrote:
> VMX already does so if the host has SMEP, in order to support the combination of
> CR0.WP=1 and CR4.SMEP=1.  However, it is perfectly safe to always do so, and in
> fact VMX already ends up running with EFER.NXE=1 on old processors that lack the
> "load EFER" controls, because it may help avoiding a slow MSR write.  Removing
> all the conditionals simplifies the code.
> 
> SVM does not have similar code, but it should since recent AMD processors do
> support SMEP.  So this patch also makes the code for the two vendors more similar
> while fixing NPT=0, CR0.WP=1 and CR4.SMEP=1 on AMD processors.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Joerg Roedel <jroedel@xxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Looks good to me.

Reviewed-by: Joerg Roedel <jroedel@xxxxxxx>

> ---
>  arch/x86/kvm/svm.c     | 10 ++++++++--
>  arch/x86/kvm/vmx/vmx.c | 14 +++-----------
>  2 files changed, 11 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index b6feb6a11a8d..2c452293c7cc 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -732,8 +732,14 @@ static int get_npt_level(struct kvm_vcpu *vcpu)
>  static void svm_set_efer(struct kvm_vcpu *vcpu, u64 efer)
>  {
>  	vcpu->arch.efer = efer;
> -	if (!npt_enabled && !(efer & EFER_LMA))
> -		efer &= ~EFER_LME;
> +
> +	if (!npt_enabled) {
> +		/* Shadow paging assumes NX to be available.  */
> +		efer |= EFER_NX;
> +
> +		if (!(efer & EFER_LMA))
> +			efer &= ~EFER_LME;
> +	}
>  
>  	to_svm(vcpu)->vmcb->save.efer = efer | EFER_SVME;
>  	mark_dirty(to_svm(vcpu)->vmcb, VMCB_CR);
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 2a2ba277c676..e191d41afb34 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -896,17 +896,9 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
>  	u64 guest_efer = vmx->vcpu.arch.efer;
>  	u64 ignore_bits = 0;
>  
> -	if (!enable_ept) {
> -		/*
> -		 * NX is needed to handle CR0.WP=1, CR4.SMEP=1.  Testing
> -		 * host CPUID is more efficient than testing guest CPUID
> -		 * or CR4.  Host SMEP is anyway a requirement for guest SMEP.
> -		 */
> -		if (boot_cpu_has(X86_FEATURE_SMEP))
> -			guest_efer |= EFER_NX;
> -		else if (!(guest_efer & EFER_NX))
> -			ignore_bits |= EFER_NX;
> -	}
> +	/* Shadow paging assumes NX to be available.  */
> +	if (!enable_ept)
> +		guest_efer |= EFER_NX;
>  
>  	/*
>  	 * LMA and LME handled by hardware; SCE meaningless outside long mode.
> -- 
> 2.21.0