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

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Message-ID: <YKQiTlDG1sZ4Zd2E@google.com>
Date:   Tue, 18 May 2021 20:23:42 +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 Mon, May 17, 2021, Reiji Watanabe wrote:
> >  void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> >  {
> > +       unsigned long old_cr0 = kvm_read_cr0(vcpu);
> > +       unsigned long old_cr4 = kvm_read_cr4(vcpu);
> > +
> >         kvm_lapic_reset(vcpu, init_event);
> >
> >         vcpu->arch.hflags = 0;
> > @@ -10483,6 +10485,10 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> >         vcpu->arch.ia32_xss = 0;
> >
> >         static_call(kvm_x86_vcpu_reset)(vcpu, init_event);
> > +
> > +       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.