From mboxrd@z Thu Jan  1 00:00:00 1970
From: Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH] Use WARN_ON_ONCE for missing X86_FEATURE_NRIPS
Date: Tue, 6 Oct 2015 12:28:38 +0200
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Cc: Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
	Dirk =?iso-8859-1?Q?M=FCller?= <dmueller@suse.com>
To: Bandan Das <bsd@redhat.com>
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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
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