linux-kernel - Re: [PATCH 06/43] KVM: x86: Properly reset MMU context at vCPU RESET/INIT

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Date:   Wed, 19 May 2021 17:16:18 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Reiji Watanabe <reijiw@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/43] KVM: x86: Properly reset MMU context at vCPU
 RESET/INIT

On Tue, May 18, 2021, Reiji Watanabe wrote:
> > > > +       if (kvm_cr0_mmu_role_changed(old_cr0, kvm_read_cr0(vcpu)) ||
> > > > +           kvm_cr4_mmu_role_changed(old_cr4, kvm_read_cr4(vcpu)))
> > > > +               kvm_mmu_reset_context(vcpu);
> > > >  }
> > >
> > > I'm wondering if kvm_vcpu_reset() should call kvm_mmu_reset_context()
> > > for a change in EFER.NX as well.
> >
> > Oooh.  So there _should_ be no need.   Paging has to be enabled for EFER.NX to
> > be relevant, and INIT toggles CR0.PG 1=>0 if paging was enabled and so is
> > guaranteed to trigger a context reset.  And we do want to skip the context reset,
> > e.g. INIT-SIPI-SIPI when the vCPU has paging disabled should continue using the
> > same MMU.
> >
> > But, kvm_calc_mmu_role_common() neglects to ignore NX if CR0.PG=0, and so the
> > MMU role will be stale if INIT clears EFER.NX without forcing a context reset.
> > However, that's benign from a functionality perspective because the context
> > itself correctly incorporates CR0.PG, it's only the role that's borked.  I.e.
> > KVM will fail to reuse a page/context due to the spurious role.nxe, but the
> > permission checks are always be correct.
> >
> > I'll add a comment here and send a patch to fix the role calculation.
> 
> Thank you so much for the explanation !
> I understand your intention and why it would be benign.
> 
> Then, I'm wondering if kvm_cr4_mmu_role_changed() needs to be
> called here.  Looking at the Intel SDM, in my understanding,
> all the bits kvm_cr4_mmu_role_changed() checks are relevant
> only if paging is enabled.  (Or is my understanding incorrect ??)

Duh, yes.  And it goes even beyond that, CR0.WP is only relevant if CR0.PG=1,
i.e. INIT with CR0.PG=0 and CR0.WP=1 will incorrectly trigger a MMU reset with
the current logic.

Sadly, simply omitting the CR4 check puts us in an awkward situation where, due
to the MMU role CR4 calculations not accounting for CR0.PG=0, KVM will run with
a stale role.

The other consideration is that kvm_post_set_cr4() and kvm_post_set_cr0() should
also skip kvm_mmu_reset_context() if CR0.PG=0, but again that requires fixing
the role calculations first (or at the same time).

I think I'll throw in those cleanups to the beginning of this series.  The result
is going to be disgustingly long, but I really don't want to introduce code that
knowingly leaves KVM in an inconsistent state, nor do I want to add useless
checks on CR4 and EFER.