[PATCH] KVM: nSVM: Check for NRIPS support before updating control field

[PATCH] KVM: nSVM: Check for NRIPS support before updating control field

[Bandan Das]

If hardware doesn't support DecodeAssist - a feature that provides
more information about the intercept in the VMCB, KVM decodes the
instruction and then updates the next_rip vmcb control field.
However, NRIP support itself depends on cpuid Fn8000_000A_EDX[NRIPS].
Since skip_emulated_instruction() doesn't verify nrip support
before accepting control.next_rip as valid, avoid writing this
field if support isn't present.

Signed-off-by: Bandan Das <bsd@redhat.com>
---
 arch/x86/kvm/svm.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 9afa233..4911bf1 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -511,8 +511,10 @@ static void skip_emulated_instruction(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 
-	if (svm->vmcb->control.next_rip != 0)
+	if (svm->vmcb->control.next_rip != 0) {
+		WARN_ON(!static_cpu_has(X86_FEATURE_NRIPS));
 		svm->next_rip = svm->vmcb->control.next_rip;
+	}
 
 	if (!svm->next_rip) {
 		if (emulate_instruction(vcpu, EMULTYPE_SKIP) !=
@@ -4317,7 +4319,9 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
 		break;
 	}
 
-	vmcb->control.next_rip  = info->next_rip;
+	/* TODO: Advertise NRIPS to guest hypervisor unconditionally */
+	if (static_cpu_has(X86_FEATURE_NRIPS))
+		vmcb->control.next_rip  = info->next_rip;
 	vmcb->control.exit_code = icpt_info.exit_code;
 	vmexit = nested_svm_exit_handled(svm);
 
-- 
2.1.0

Re: [PATCH] KVM: nSVM: Check for NRIPS support before updating control field

[Paolo Bonzini]

On 11/06/2015 08:05, Bandan Das wrote:
> 
> If hardware doesn't support DecodeAssist - a feature that provides
> more information about the intercept in the VMCB, KVM decodes the
> instruction and then updates the next_rip vmcb control field.
> However, NRIP support itself depends on cpuid Fn8000_000A_EDX[NRIPS].
> Since skip_emulated_instruction() doesn't verify nrip support
> before accepting control.next_rip as valid, avoid writing this
> field if support isn't present.
> 
> Signed-off-by: Bandan Das <bsd@redhat.com>
> ---
>  arch/x86/kvm/svm.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 9afa233..4911bf1 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -511,8 +511,10 @@ static void skip_emulated_instruction(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  
> -	if (svm->vmcb->control.next_rip != 0)
> +	if (svm->vmcb->control.next_rip != 0) {
> +		WARN_ON(!static_cpu_has(X86_FEATURE_NRIPS));
>  		svm->next_rip = svm->vmcb->control.next_rip;
> +	}
>  
>  	if (!svm->next_rip) {
>  		if (emulate_instruction(vcpu, EMULTYPE_SKIP) !=
> @@ -4317,7 +4319,9 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
>  		break;
>  	}
>  
> -	vmcb->control.next_rip  = info->next_rip;
> +	/* TODO: Advertise NRIPS to guest hypervisor unconditionally */
> +	if (static_cpu_has(X86_FEATURE_NRIPS))
> +		vmcb->control.next_rip  = info->next_rip;
>  	vmcb->control.exit_code = icpt_info.exit_code;
>  	vmexit = nested_svm_exit_handled(svm);
>  
> 

Applied, thanks.

paolo