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From:   Jim Mattson <jmattson@google.com>
Date:   Mon, 23 Mar 2020 09:36:22 -0700
Message-ID: <CALMp9eR42eM7g81EgHieyNky+kP2mycO7UyMN+y2ibLoqrD2Yg@mail.gmail.com>
Subject: Re: [PATCH v3 03/37] KVM: nVMX: Invalidate all EPTP contexts when
 emulating INVEPT for L1
To:     Sean Christopherson <sean.j.christopherson@intel.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>,
        Ben Gardon <bgardon@google.com>,
        Junaid Shahid <junaids@google.com>,
        Liran Alon <liran.alon@oracle.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        John Haxby <john.haxby@oracle.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Tom Lendacky <thomas.lendacky@amd.com>
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On Mon, Mar 23, 2020 at 9:28 AM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Mon, Mar 23, 2020 at 09:24:25AM -0700, Jim Mattson wrote:
> > On Fri, Mar 20, 2020 at 2:29 PM Sean Christopherson
> > <sean.j.christopherson@intel.com> wrote:
> > >
> > > Free all L2 (guest_mmu) roots when emulating INVEPT for L1.  Outstanding
> > > changes to the EPT tables managed by L1 need to be recognized, and
> > > relying on KVM to always flush L2's EPTP context on nested VM-Enter is
> > > dangerous.
> > >
> > > Similar to handle_invpcid(), rely on kvm_mmu_free_roots() to do a remote
> > > TLB flush if necessary, e.g. if L1 has never entered L2 then there is
> > > nothing to be done.
> > >
> > > Nuking all L2 roots is overkill for the single-context variant, but it's
> > > the safe and easy bet.  A more precise zap mechanism will be added in
> > > the future.  Add a TODO to call out that KVM only needs to invalidate
> > > affected contexts.
> > >
> > > Fixes: b119019847fbc ("kvm: nVMX: Remove unnecessary sync_roots from handle_invept")
> >
> > The bug existed well before the commit indicated in the "Fixes" line.
>
> Ah, my bad.  A cursory glance at commit b119019847fbc makes that quite
> obvious.  This should be
>
>   Fixes: bfd0a56b9000 ("nEPT: Nested INVEPT")

Actually, I think that things were fine back then (though we
gratuitously flushed L1's TLB as a result of an emulated INVEPT). The
problem started when we stopped flushing the TLB on every emulated
VM-entry (i.e. L1 -> L2 transitions). I'm not sure what that commit
was, but I think you referenced it in an earlier email.