Re: [PATCH v2 15/18] KVM: x86/mmu: rename kvm_mmu_new_pgd, introduce variant that calls get_guest_pgd

Maxim Levitsky
2022-02-24T15:41:49Z

On Fri, 2022-02-18 at 21:00 +0000, Sean Christopherson wrote:
> On Fri, Feb 18, 2022, Paolo Bonzini wrote:
> > On 2/17/22 22:03, Paolo Bonzini wrote:
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index adcee7c305ca..9800c8883a48 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -1189,7 +1189,7 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
> > > return 1;
> > > if (cr3 != kvm_read_cr3(vcpu))
> > > - kvm_mmu_new_pgd(vcpu, cr3);
> > > + kvm_mmu_update_root(vcpu);
> > > vcpu->arch.cr3 = cr3;
> > > kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
> >
> > Uh-oh, this has to become:
> >
> > vcpu->arch.cr3 = cr3;
> > kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
> > if (!is_pae_paging(vcpu))
> > kvm_mmu_update_root(vcpu);
> >
> > The regression would go away after patch 16, but this is more tidy apart
> > from having to check is_pae_paging *again*.
> >
> > Incremental patch:
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index adcee7c305ca..0085e9fba372 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1188,11 +1189,11 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
> > if (is_pae_paging(vcpu) && !load_pdptrs(vcpu, cr3))
> > return 1;
> > - if (cr3 != kvm_read_cr3(vcpu))
> > - kvm_mmu_update_root(vcpu);
> > -
> > vcpu->arch.cr3 = cr3;
> > kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
> > + if (!is_pae_paging(vcpu))
> > + kvm_mmu_update_root(vcpu);
> > +
> > /* Do not call post_set_cr3, we do not get here for confidential guests. */
> >
> > An alternative is to move the vcpu->arch.cr3 update in load_pdptrs.
> > Reviewers, let me know if you prefer that, then I'll send v3.
>
> c) None of the above.
>
> MOV CR3 never requires a new root if TDP is enabled, and the guest_mmu is used if
> and only if TDP is enabled. Even when KVM intercepts CR3 when EPT=1 && URG=0, it
> does so only to snapshot vcpu->arch.cr3, there's no need to get a new PGD.
>
> Unless I'm missing something, your original suggestion of checking tdp_enabled is
> the way to go.
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 6e0f7f22c6a7..2b02029c63d0 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1187,7 +1187,7 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
> if (is_pae_paging(vcpu) && !load_pdptrs(vcpu, cr3))
> return 1;
>
> - if (cr3 != kvm_read_cr3(vcpu))
> + if (!tdp_enabled && cr3 != kvm_read_cr3(vcpu))
> kvm_mmu_new_pgd(vcpu, cr3);
>
> vcpu->arch.cr3 = cr3;
>
>

Is this actually related to the discussion? The original issue that Paolo found in his patch
was that kvm_mmu_update_root now reads _current_ cr3, thus it has to be set before calling it.

I do agree that kvm_set_cr3 doesn't need to do anything when TDP is enabled, this is a different
issue which doesn't cause much harm (the fast_pgd_switch with direct roots will reuse current root),
but still would raise KVM_REQ_LOAD_MMU_PGD without need for it and such.

About the patch itself, other than this mentioned issue, it looks fine to me.

Best regards,
Maxim Levitsky