Re: [PATCH] kvm: svm: Only propagate next_rip when guest supports it

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/10/2015 11:51, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
> 
> Currently we always write the next_rip of the shadow vmcb to
> the guests vmcb when we emulate a vmexit. This could confuse
> the guest when its cpuid indicated no support for the
> next_rip feature.
> 
> Fix this by only propagating next_rip if the guest actually
> supports it.
> 
> Cc: Bandan Das <bsd@redhat.com>
> Cc: Dirk Mueller <dmueller@suse.com>
> Tested-by: Dirk Mueller <dmueller@suse.com>
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
>  arch/x86/kvm/cpuid.h | 21 +++++++++++++++++++++
>  arch/x86/kvm/svm.c   |  4 +++-
>  2 files changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index dd05b9c..effca1f 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -133,4 +133,25 @@ static inline bool guest_cpuid_has_mpx(struct kvm_vcpu *vcpu)
>  	best = kvm_find_cpuid_entry(vcpu, 7, 0);
>  	return best && (best->ebx & bit(X86_FEATURE_MPX));
>  }
> +
> +/*
> + * NRIPS is provided through cpuidfn 0x8000000a.edx bit 3
> + */
> +#define BIT_NRIPS	3
> +
> +static inline bool guest_cpuid_has_nrips(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_cpuid_entry2 *best;
> +
> +	best = kvm_find_cpuid_entry(vcpu, 0x8000000a, 0);
> +
> +	/*
> +	 * NRIPS is a scattered cpuid feature, so we can't use
> +	 * X86_FEATURE_NRIPS here (X86_FEATURE_NRIPS would be bit
> +	 * position 8, not 3).
> +	 */
> +	return best && (best->edx & bit(BIT_NRIPS));
> +}
> +#undef BIT_NRIPS
> +
>  #endif
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 2f9ed1f..4084b33 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -2365,7 +2365,9 @@ static int nested_svm_vmexit(struct vcpu_svm *svm)
>  	nested_vmcb->control.exit_info_2       = vmcb->control.exit_info_2;
>  	nested_vmcb->control.exit_int_info     = vmcb->control.exit_int_info;
>  	nested_vmcb->control.exit_int_info_err = vmcb->control.exit_int_info_err;
> -	nested_vmcb->control.next_rip          = vmcb->control.next_rip;
> +
> +	if (guest_cpuid_has_nrips(&svm->vcpu))

This could be a bit expensive to do on every vmexit.  Can you benchmark
it with kvm-unit-tests, or just cache the result in struct vcpu_svm?

Thanks,

Paolo

> +		nested_vmcb->control.next_rip  = vmcb->control.next_rip;
>  
>  	/*
>  	 * If we emulate a VMRUN/#VMEXIT in the same host #vmexit cycle we have
>