On Wed, May 5, 2021 at 9:28 AM Ben Gardon <bgardon@xxxxxxxxxx> wrote:
>
> On Wed, May 5, 2021 at 2:38 AM Kai Huang <kai.huang@xxxxxxxxx> 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.
I went through and manually audited the code. I think the only case
that needs to be added to this is for nx recovery:
--- 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,
Otherwise we won't zap the full page with this change, resulting in
ineffective or less reliable NX recovery.
>
> >
> > Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
> > ---
> > 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
> >
