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Date:   Tue, 15 Jun 2021 23:33:39 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Jim Mattson <jmattson@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/4] KVM: x86: Simplify logic to handle lack of host NX
 support
Message-ID: <YMk40yLeyV1DHpYp@google.com>
References: <20210615164535.2146172-1-seanjc@google.com>
 <20210615164535.2146172-5-seanjc@google.com>
 <CALMp9eRGj_5+dZXQazVEkeKeDnc7GFm1Vnt2RS_V6akAR=rZsA@mail.gmail.com>
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On Tue, Jun 15, 2021, Jim Mattson wrote:
> On Tue, Jun 15, 2021 at 9:45 AM Sean Christopherson <seanjc@google.com> wrote:
> > @@ -226,7 +224,7 @@ static void cpuid_fix_nx_cap(struct kvm_vcpu *vcpu)
> >                         break;
> >                 }
> >         }
> > -       if (entry && cpuid_entry_has(entry, X86_FEATURE_NX) && !is_efer_nx()) {
> > +       if (entry && cpuid_entry_has(entry, X86_FEATURE_NX)) {
> >                 cpuid_entry_clear(entry, X86_FEATURE_NX);
> >                 printk(KERN_INFO "kvm: guest NX capability removed\n");
> >         }
> 
> It would be nice if we chose one consistent approach to dealing with
> invalid guest CPUID information and stuck with it. Silently modifying
> the table provided by userspace seems wrong to me. I much prefer the
> kvm_check_cpuid approach of telling userspace that the guest CPUID
> information is invalid. (Of course, once we return -EINVAL for more
> than one field, good luck figuring out which field is invalid!)

Yeah.  I suspect this one can be dropped if EFER.NX is required for everything
except EPT, but I didn't fully grok the problem that this was fixing, and it's
such an esoteric case that I both don't care and am terrified of breaking some
bizarre case.