linux-kernel - Re: [PATCH 6/9] KVM: SVM: WARN if KVM attempts emulation on #UD or #GP for SEV guests

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Message-ID: <YemWCwhQ8aYcqUw9@google.com>
Date:   Thu, 20 Jan 2022 17:04:11 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Liam Merwick <liam.merwick@...cle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH 6/9] KVM: SVM: WARN if KVM attempts emulation on #UD or
 #GP for SEV guests

On Thu, Jan 20, 2022, Liam Merwick wrote:
> On 20/01/2022 01:07, Sean Christopherson wrote:
> > WARN if KVM attempts to emulate in response to #UD or #GP for SEV guests,
> > i.e. if KVM intercepts #UD or #GP, as emulation on any fault except #NPF
> > is impossible since KVM cannot read guest private memory to get the code
> > stream, and the CPU's DecodeAssists feature only provides the instruction
> > bytes on #NPF.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >   arch/x86/kvm/svm/svm.c | 3 +++
> >   1 file changed, 3 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 994224ae2731..ed2ca875b84b 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -4267,6 +4267,9 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
> >   	if (!sev_guest(vcpu->kvm))
> >   		return true;
> > +	/* #UD and #GP should never be intercepted for SEV guests. */
> > +	WARN_ON_ONCE(emul_type & (EMULTYPE_TRAP_UD | EMULTYPE_VMWARE_GP));
> 
> What about EMULTYPE_TRAP_UD_FORCED?

Hmm, yeah, it's worth adding, there's no additional cost.  I was thinking it was
a modifier to EMULTYPE_TRAP_UD, but it's a replacement specifically to bypass
the EmulateOnUD check (which I should have remembered since I added the type...).