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@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.
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 ended up with below. Is it OK to you?
If you still think a comment of level should be added, would you be more specific so that
I can add it?
------------------------------------------------------------------------
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@xxxxxxxxx>
---
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@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
> >
