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From:   Jim Mattson <jmattson@google.com>
Date:   Thu, 7 Jul 2022 16:12:44 -0700
Message-ID: <CALMp9eTjVksjTkOQ-=HNKB2oyuCcDw3mC=E+P8XAXafy2MmrPA@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 2:39 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Thu, Jul 07, 2022, Jim Mattson wrote:
> > 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.
>
> Actually, they're still required if unrestricted guest isn't supported.
>
> > > 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.
>
> Fair enough, I was using "EPT" in the "KVM is using EPT" sense.  But even that's
> wrong as KVM intercepts CR3 accesses when EPT is enabled, but unrestricted guest
> is disabled and the guest disables paging.

MOV-to-CR3 is also a required intercept for allow_smaller_maxphyaddr,
when the guest is in PAE mode. So, that one, at least, isn't anywhere
near obsolete. :-)

> Vitaly, since the CR3 fields are still technically "needed", maybe just be
> explicit?
>
>   KVM: VMX: Adjust CR3/INVPLG interception for EPT=y at runtime, not setup