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Subject[PATCH 3/4] KVM: x86/mmu: Don't bottom out on leafs when zapping collapsible SPTEs
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When zapping collapsible SPTEs in the TDP MMU, don't bottom out on a leaf
SPTE now that KVM doesn't require a PFN to compute the host mapping level,
i.e. now that there's no need to first find a leaf SPTE and then step
back up.

Drop the now unused tdp_iter_step_up(), as it is not the safest of
helpers (using any of the low level iterators requires some understanding
of the various side effects).

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/tdp_iter.c | 9 ------
arch/x86/kvm/mmu/tdp_iter.h | 1 -
arch/x86/kvm/mmu/tdp_mmu.c | 57 ++++++++++++++++++-------------------
3 files changed, 27 insertions(+), 40 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_iter.c b/arch/x86/kvm/mmu/tdp_iter.c
index 9c65a64a56d9..39b48e7d7d1a 100644
--- a/arch/x86/kvm/mmu/tdp_iter.c
+++ b/arch/x86/kvm/mmu/tdp_iter.c
@@ -145,15 +145,6 @@ static bool try_step_up(struct tdp_iter *iter)
return true;
}

-/*
- * Step the iterator back up a level in the paging structure. Should only be
- * used when the iterator is below the root level.
- */
-void tdp_iter_step_up(struct tdp_iter *iter)
-{
- WARN_ON(!try_step_up(iter));
-}
-
/*
* Step to the next SPTE in a pre-order traversal of the paging structure.
* To get to the next SPTE, the iterator either steps down towards the goal
diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
index adfca0cf94d3..f0af385c56e0 100644
--- a/arch/x86/kvm/mmu/tdp_iter.h
+++ b/arch/x86/kvm/mmu/tdp_iter.h
@@ -114,6 +114,5 @@ void tdp_iter_start(struct tdp_iter *iter, struct kvm_mmu_page *root,
int min_level, gfn_t next_last_level_gfn);
void tdp_iter_next(struct tdp_iter *iter);
void tdp_iter_restart(struct tdp_iter *iter);
-void tdp_iter_step_up(struct tdp_iter *iter);

#endif /* __KVM_X86_MMU_TDP_ITER_H */
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index d75d93edc40a..40ccb5fba870 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1721,10 +1721,6 @@ void kvm_tdp_mmu_clear_dirty_pt_masked(struct kvm *kvm,
clear_dirty_pt_masked(kvm, root, gfn, mask, wrprot);
}

-/*
- * Clear leaf entries which could be replaced by large mappings, for
- * GFNs within the slot.
- */
static void zap_collapsible_spte_range(struct kvm *kvm,
struct kvm_mmu_page *root,
const struct kvm_memory_slot *slot)
@@ -1736,48 +1732,49 @@ static void zap_collapsible_spte_range(struct kvm *kvm,

rcu_read_lock();

- tdp_root_for_each_pte(iter, root, start, end) {
+ for_each_tdp_pte_min_level(iter, root, PG_LEVEL_2M, start, end) {
+retry:
if (tdp_mmu_iter_cond_resched(kvm, &iter, false, true))
continue;

- if (!is_shadow_present_pte(iter.old_spte) ||
- !is_last_spte(iter.old_spte, iter.level))
+ if (iter.level > KVM_MAX_HUGEPAGE_LEVEL ||
+ !is_shadow_present_pte(iter.old_spte))
+ continue;
+
+ /*
+ * Don't zap leaf SPTEs, if a leaf SPTE could be replaced with
+ * a large page size, then its parent would have been zapped
+ * instead of stepping down.
+ */
+ if (is_last_spte(iter.old_spte, iter.level))
+ continue;
+
+ /*
+ * If iter.gfn resides outside of the slot, i.e. the page for
+ * the current level overlaps but is not contained by the slot,
+ * then the SPTE can't be made huge. More importantly, trying
+ * to query that info from slot->arch.lpage_info will cause an
+ * out-of-bounds access.
+ */
+ if (iter.gfn < start || iter.gfn >= end)
continue;

max_mapping_level = kvm_mmu_max_mapping_level(kvm, slot,
iter.gfn, PG_LEVEL_NUM);
-
- WARN_ON(max_mapping_level < iter.level);
-
- /*
- * If this page is already mapped at the highest
- * viable level, there's nothing more to do.
- */
- if (max_mapping_level == iter.level)
+ if (max_mapping_level < iter.level)
continue;

- /*
- * The page can be remapped at a higher level, so step
- * up to zap the parent SPTE.
- */
- while (max_mapping_level > iter.level)
- tdp_iter_step_up(&iter);
-
/* Note, a successful atomic zap also does a remote TLB flush. */
- tdp_mmu_zap_spte_atomic(kvm, &iter);
-
- /*
- * If the atomic zap fails, the iter will recurse back into
- * the same subtree to retry.
- */
+ if (tdp_mmu_zap_spte_atomic(kvm, &iter))
+ goto retry;
}

rcu_read_unlock();
}

/*
- * Clear non-leaf entries (and free associated page tables) which could
- * be replaced by large mappings, for GFNs within the slot.
+ * Zap non-leaf SPTEs (and free their associated page tables) which could
+ * be replaced by huge pages, for GFNs within the slot.
*/
void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
const struct kvm_memory_slot *slot)
--
2.37.0.170.g444d1eabd0-goog
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