Re: [PATCH] Use WARN_ON_ONCE for missing X86_FEATURE_NRIPS

[Bandan Das <bsd@redhat.com>]

Joerg Roedel <joro@8bytes.org> writes:

> On Thu, Oct 01, 2015 at 06:31:27PM -0400, Bandan Das wrote:
>> >> @@ -514,7 +514,7 @@ static void skip_emulated_instruction(struct kvm_vcpu *vcpu)
>> >>  	struct vcpu_svm *svm = to_svm(vcpu);
>> >>  
>> >>  	if (svm->vmcb->control.next_rip != 0) {
>> >> -		WARN_ON(!static_cpu_has(X86_FEATURE_NRIPS));
>> >> +		WARN_ON_ONCE(!static_cpu_has(X86_FEATURE_NRIPS));
>> >>  		svm->next_rip = svm->vmcb->control.next_rip;
>> >>  	}
>
> I looked again how this could possibly be triggered, and I am somewhat
> confused now.
>
> So svm->vmcb->control.next_rip is only written by hardware or in
> svm_check_intercept(). Both cases write only to this field, if the
> hardware supports X86_FEATURE_NRIPS. The write in nested_svm_vmexit only

Not until commit f104765b4f81fd74d69e0eb161e89096deade2db. So, an older L1
kernel will trigger it.

> targets the guests VMCB, and we don't use that one again.
>
> So I can't see how the WARN_ON above could be triggered. Do I miss
> something or might this also be a miscompilation of static_cpu_has?
>
>
> 	Joerg