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

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Date:   Tue, 18 May 2021 15:42:02 -0700
From:   Reiji Watanabe <reijiw@...gle.com>
To:     Sean Christopherson <seanjc@...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

> > > +       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 ??)

Thanks,
Reiji