linux-kernel - Re: [PATCH v3 03/37] KVM: nVMX: Invalidate all EPTP contexts when emulating INVEPT for L1

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Date:   Mon, 23 Mar 2020 09:44:48 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ben Gardon <bgardon@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        John Haxby <john.haxby@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v3 03/37] KVM: nVMX: Invalidate all EPTP contexts when
 emulating INVEPT for L1

On Mon, Mar 23, 2020 at 09:36:22AM -0700, Jim Mattson wrote:
> On Mon, Mar 23, 2020 at 9:28 AM Sean Christopherson
> <sean.j.christopherson@...el.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@...el.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.

Hmm, true.  I was thinking it was the original commit because it didn't
operate on guest_mmu, but guest_mmu didn't exist back then.  So I think

  Fixes: 14c07ad89f4d ("x86/kvm/mmu: introduce guest_mmu")

would be appropriate?