Re: [PATCH v2] KVM: nSVM: Never use L0's PAUSE loop exiting while L2 is running

[Yosry Ahmed]

> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index e7fdd7a9c280..ac21f402c1ca 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -913,7 +913,12 @@ static void grow_ple_window(struct kvm_vcpu *vcpu)
>  	struct vmcb_control_area *control = &svm->vmcb->control;
>  	int old = control->pause_filter_count;
>  
> -	if (kvm_pause_in_guest(vcpu->kvm))
> +	/*
> +	 * While running L2, KVM should intercept PAUSE if and only if L1 wants
> +	 * to intercept PAUSE, and L1's intercept should take priority, i.e.
> +	 * KVM should never handle a PAUSE intercept from L2.
> +	 */
> +	if (WARN_ON_ONCE(is_guest_mode(vcpu) || kvm_pause_in_guest(vcpu->kvm)))
>  		return;

I initially thought putting kvm_pause_in_guest() inside the
WARN_ON_ONCE() was a mistake, but I don't think it is. KVM only
intercepts PAUSE if kvm_pause_in_guest() is false, so grow_ple_window()
should never be called.

But I think this applies even without this patch, right? Maybe would be
clearer to do WARN_ON_ONCE(kvm_pause_in_guest()) in one commit and then
add is_guest_mode() here?

>  
>  	control->pause_filter_count = __grow_ple_window(old,
> @@ -934,7 +939,7 @@ static void shrink_ple_window(struct kvm_vcpu *vcpu)
>  	struct vmcb_control_area *control = &svm->vmcb->control;
>  	int old = control->pause_filter_count;
>  
> -	if (kvm_pause_in_guest(vcpu->kvm))
> +	if (is_guest_mode(vcpu))
>  		return;
>  
>  	control->pause_filter_count =
> 
> base-commit: 6d35786de28116ecf78797a62b84e6bf3c45aa5a
> -- 
> 2.54.0.563.g4f69b47b94-goog
> 
>