Re: [PATCH 3/3] KVM: x86/mmu: Fix TDP MMU page table level

[Ben Gardon <bgardon@google.com>]

On Thu, May 6, 2021 at 1:00 AM Kai Huang <kai.huang@intel.com> wrote:
>
> On Wed, 2021-05-05 at 09:28 -0700, Ben Gardon wrote:
> > On Wed, May 5, 2021 at 2:38 AM Kai Huang <kai.huang@intel.com> wrote:
> > >
> > > TDP MMU iterator's level is identical to page table's actual level.  For
> > > instance, for the last level page table (whose entry points to one 4K
> > > page), iter->level is 1 (PG_LEVEL_4K), and in case of 5 level paging,
> > > the iter->level is mmu->shadow_root_level, which is 5.  However, struct
> > > kvm_mmu_page's level currently is not set correctly when it is allocated
> > > in kvm_tdp_mmu_map().  When iterator hits non-present SPTE and needs to
> > > allocate a new child page table, currently iter->level, which is the
> > > level of the page table where the non-present SPTE belongs to, is used.
> > > This results in struct kvm_mmu_page's level always having its parent's
> > > level (excpet root table's level, which is initialized explicitly using
> > > mmu->shadow_root_level).  This is kinda wrong, and not consistent with
> > > existing non TDP MMU code.  Fortuantely the sp->role.level is only used
> > > in handle_removed_tdp_mmu_page(), which apparently is already aware of
> > > this, and handles correctly.  However to make it consistent with non TDP
> > > MMU code (and fix the issue that both root page table and any child of
> > > it having shadow_root_level), fix this by using iter->level - 1 in
> > > kvm_tdp_mmu_map().  Also modify handle_removed_tdp_mmu_page() to handle
> > > such change.
> >
> > Ugh. Thank you for catching this. This is going to take me a bit to
> > review as I should audit the code more broadly for this problem in the
> > TDP MMU.
> > It would probably also be a good idea to add a comment on the level
> > field to say that it represents the level of the SPTEs in the
> > associated page, not the level of the SPTE that links to the
> > associated page.
> > Hopefully that will prevent similar future misunderstandings.
>
> Regarding to adding  a comment, sorry I had a hard time to figure out where to add. Did
> you mean level field of 'struct kvm_mmu_page_role', or 'struct tdp_iter'? If it is the
> former, to me not quite useful.

I meant the level field of 'struct kvm_mmu_page_role', but if you
don't think it makes sense to add one there, I don't feel strongly
either way.

>
> I ended up with below. Is it OK to you?

Yeah, it looks good to me.

>
> If you still think a comment of level should be added, would you be more specific so that
> I can add it?

struct {
+       /*
+       * The level of the SPT tracked by this SP, as opposed to the
level of the
+       * parent SPTE linking this SPT.
+        */
        unsigned level:4;

... I guess that does sound kind of unnecessary.

>
> ------------------------------------------------------------------------
>
> TDP MMU iterator's level is identical to page table's actual level.  For
> instance, for the last level page table (whose entry points to one 4K
> page), iter->level is 1 (PG_LEVEL_4K), and in case of 5 level paging,
> the iter->level is mmu->shadow_root_level, which is 5.  However, struct
> kvm_mmu_page's level currently is not set correctly when it is allocated
> in kvm_tdp_mmu_map().  When iterator hits non-present SPTE and needs to
> allocate a new child page table, currently iter->level, which is the
> level of the page table where the non-present SPTE belongs to, is used.
> This results in struct kvm_mmu_page's level always having its parent's
> level (excpet root table's level, which is initialized explicitly using
> mmu->shadow_root_level).
>
> This is kinda wrong, and not consistent with existing non TDP MMU code.
> Fortuantely sp->role.level is only used in handle_removed_tdp_mmu_page()
> and kvm_tdp_mmu_zap_sp(), and they are already aware of this and behave
> correctly.  However to make it consistent with legacy MMU code (and fix
> the issue that both root page table and its child page table have
> shadow_root_level), use iter->level - 1 in kvm_tdp_mmu_map(), and change
> handle_removed_tdp_mmu_page() and kvm_tdp_mmu_zap_sp() accordingly.
>
> Signed-off-by: Kai Huang <kai.huang@intel.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 8 ++++----
>  arch/x86/kvm/mmu/tdp_mmu.h | 2 +-
>  2 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 5e28fbabcd35..45fb889f6a94 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -335,7 +335,7 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, tdp_ptep_t
> pt,
>
>         for (i = 0; i < PT64_ENT_PER_PAGE; i++) {
>                 sptep = rcu_dereference(pt) + i;
> -               gfn = base_gfn + (i * KVM_PAGES_PER_HPAGE(level - 1));
> +               gfn = base_gfn + i * KVM_PAGES_PER_HPAGE(level);
>
>                 if (shared) {
>                         /*
> @@ -377,12 +377,12 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, tdp_ptep_t
> pt,
>                         WRITE_ONCE(*sptep, REMOVED_SPTE);
>                 }
>                 handle_changed_spte(kvm, kvm_mmu_page_as_id(sp), gfn,
> -                                   old_child_spte, REMOVED_SPTE, level - 1,
> +                                   old_child_spte, REMOVED_SPTE, level,
>                                     shared);
>         }
>
>         kvm_flush_remote_tlbs_with_address(kvm, gfn,
> -                                          KVM_PAGES_PER_HPAGE(level));
> +                                          KVM_PAGES_PER_HPAGE(level + 1));
>
>         call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
>  }
> @@ -1013,7 +1013,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, gpa_t gpa, u32
> error_code,
>                 }
>
>                 if (!is_shadow_present_pte(iter.old_spte)) {
> -                       sp = alloc_tdp_mmu_page(vcpu, iter.gfn, iter.level);
> +                       sp = alloc_tdp_mmu_page(vcpu, iter.gfn, iter.level - 1);
>                         child_pt = sp->spt;
>
>                         new_spte = make_nonleaf_spte(child_pt,
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> index 5fdf63090451..7f9974c5d0b4 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.h
> +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> @@ -31,7 +31,7 @@ static inline bool kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id,
>  }
>  static inline bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
>  {
> -       gfn_t end = sp->gfn + KVM_PAGES_PER_HPAGE(sp->role.level);
> +       gfn_t end = sp->gfn + KVM_PAGES_PER_HPAGE(sp->role.level + 1);
>
>         /*
>          * Don't allow yielding, as the caller may have a flush pending.  Note,
> --
> 2.31.1
>
>
> >
> > >
> > > Signed-off-by: Kai Huang <kai.huang@intel.com>
> > > ---
> > >  arch/x86/kvm/mmu/tdp_mmu.c | 8 ++++----
> > >  1 file changed, 4 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > > index debe8c3ec844..bcfb87e1c06e 100644
> > > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > > @@ -335,7 +335,7 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, tdp_ptep_t pt,
> > >
> > >         for (i = 0; i < PT64_ENT_PER_PAGE; i++) {
> > >                 sptep = rcu_dereference(pt) + i;
> > > -               gfn = base_gfn + (i * KVM_PAGES_PER_HPAGE(level - 1));
> > > +               gfn = base_gfn + i * KVM_PAGES_PER_HPAGE(level);
> > >
> > >                 if (shared) {
> > >                         /*
> > > @@ -377,12 +377,12 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, tdp_ptep_t pt,
> > >                         WRITE_ONCE(*sptep, REMOVED_SPTE);
> > >                 }
> > >                 handle_changed_spte(kvm, kvm_mmu_page_as_id(sp), gfn,
> > > -                                   old_child_spte, REMOVED_SPTE, level - 1,
> > > +                                   old_child_spte, REMOVED_SPTE, level,
> > >                                     shared);
> > >         }
> > >
> > >         kvm_flush_remote_tlbs_with_address(kvm, gfn,
> > > -                                          KVM_PAGES_PER_HPAGE(level));
> > > +                                          KVM_PAGES_PER_HPAGE(level + 1));
> > >
> > >         call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
> > >  }
> > > @@ -1009,7 +1009,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> > >                 }
> > >
> > >                 if (!is_shadow_present_pte(iter.old_spte)) {
> > > -                       sp = alloc_tdp_mmu_page(vcpu, iter.gfn, iter.level);
> > > +                       sp = alloc_tdp_mmu_page(vcpu, iter.gfn, iter.level - 1);
> > >                         child_pt = sp->spt;
> > >
> > >                         new_spte = make_nonleaf_spte(child_pt,
> > > --
> > > 2.31.1
> > >
>
>