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

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Date:	Wed, 17 Jun 2015 14:31:25 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Bandan Das <bsd@...hat.com>, kvm@...r.kernel.org
CC:	linux-kernel@...r.kernel.org, Radim Krcmar <rkrcmar@...hat.com>,
	jan.kiszka@...mens.com
Subject: Re: [PATCH] KVM: nSVM: Check for NRIPS support before updating control
 field



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@...hat.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
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