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From:   Jim Mattson <jmattson@google.com>
Date:   Thu, 7 Jul 2022 14:32:25 -0700
Message-ID: <CALMp9eTrtFd-pcEeWvyAs7eYe1R1FPvGr0pjQNP8o8F0YHhg8A@mail.gmail.com>
Subject: Re: [PATCH v2 22/28] KVM: VMX: Clear controls obsoleted by EPT at
 runtime, not setup
To:     Sean Christopherson <seanjc@google.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>, kvm@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Anirudh Rayabharam <anrayabh@linux.microsoft.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Maxim Levitsky <mlevitsk@redhat.com>,
        linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org
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On Thu, Jul 7, 2022 at 12:30 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Thu, Jul 07, 2022, Vitaly Kuznetsov wrote:
> > Jim Mattson <jmattson@google.com> writes:
> >
> > > On Wed, Jun 29, 2022 at 8:07 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> > >>
> > >> From: Sean Christopherson <seanjc@google.com>
> > >>
> > >> Clear the CR3 and INVLPG interception controls at runtime based on
> > >> whether or not EPT is being _used_, as opposed to clearing the bits at
> > >> setup if EPT is _supported_ in hardware, and then restoring them when EPT
> > >> is not used.  Not mucking with the base config will allow using the base
> > >> config as the starting point for emulating the VMX capability MSRs.
> > >>
> > >> Signed-off-by: Sean Christopherson <seanjc@google.com>
> > >> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> > > Nit: These controls aren't "obsoleted" by EPT; they're just no longer
> > > required.
>
> Isn't that the definition of "obsolete"?  They're "no longer in use" when KVM
> enables EPT.

There are still reasons to use them aside from shadow page table
maintenance. For example, malware analysis may be interested in
intercepting CR3 changes to track process context (and to
enable/disable costly monitoring). EPT doesn't render these events
"obsolete," because you can't intercept these events using EPT.

> > I'm going to update the subject line to "KVM: VMX: Clear controls
> > unneded with EPT at runtime, not setup" retaining your authorship in v3
>
> That's fine, though s/unneded/unneeded.