linux-kernel - Re: [PATCH 1/4] KVM: x86: mark synthetic SMM vmexit as SVM_EXIT

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Date:   Tue, 01 Mar 2022 19:13:00 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org
Subject: Re: [PATCH 1/4] KVM: x86: mark synthetic SMM vmexit as SVM_EXIT_SW

On Tue, 2022-03-01 at 16:31 +0000, Sean Christopherson wrote:
> On Tue, Mar 01, 2022, Maxim Levitsky wrote:
> > Use a dummy unused vmexit reason to mark the 'VM exit' that is happening
> > when kvm exits to handle SMM, which is not a real VM exit.
> 
> Why not use "62h VMEXIT_SMI"?

Because VMEXIT_SMI is real vmexit which happens when L1 intercepts #SMI
And here nested_svm_vmexit is abused to just exit guest mode without vmexit.

> 
> > This makes it a bit easier to read the KVM trace, and avoids
> > other potential problems.
> 
> What other potential problems?

The fact that we have a stale VM exit reason in vmcb without this
patch which can be in theory consumed somewhere down the road.

This stale vm exit reason also appears in the tracs which is
very misleading.

Best regards,
	Maxim Levitsky

> 
> > Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> > ---
> >  arch/x86/kvm/svm/svm.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 7038c76fa8410..c08fd7f4f3414 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -4218,7 +4218,7 @@ static int svm_enter_smm(struct kvm_vcpu *vcpu, char *smstate)
> >  	svm->vmcb->save.rsp = vcpu->arch.regs[VCPU_REGS_RSP];
> >  	svm->vmcb->save.rip = vcpu->arch.regs[VCPU_REGS_RIP];
> >  
> > -	ret = nested_svm_vmexit(svm);
> > +	ret = nested_svm_simple_vmexit(svm, SVM_EXIT_SW);
> >  	if (ret)
> >  		return ret;
> >  
> > -- 
> > 2.26.3
> >