KVM: x86/mmu: Rename slot_handle_leaf to slot_handle_level_4k

slot_handle_leaf is a misnomer because it only operates on 4K SPTEs
whereas "leaf" is used to describe any valid terminal SPTE (4K or
large page). Rename slot_handle_leaf to slot_handle_level_4k to
avoid confusion.

Making this change makes it more obvious there is a benign discrepency
between the legacy MMU and the TDP MMU when it comes to dirty logging.
The legacy MMU only iterates through 4K SPTEs when zapping for
collapsing and when clearing D-bits. The TDP MMU, on the other hand,
iterates through SPTEs on all levels.

The TDP MMU behavior of zapping SPTEs at all levels is technically
overkill for its current dirty logging implementation, which always
demotes to 4k SPTES, but both the TDP MMU and legacy MMU zap if and only
if the SPTE can be replaced by a larger page, i.e. will not spuriously
zap 2m (or larger) SPTEs. Opportunistically add comments to explain this
discrepency in the code.

Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20211019162223.3935109-1-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -5440,8 +5440,8 @@ slot_handle_level(struct kvm *kvm, const struct kvm_memory_slot *memslot,
 }
 
 static __always_inline bool
-slot_handle_leaf(struct kvm *kvm, const struct kvm_memory_slot *memslot,
-		 slot_level_handler fn, bool flush_on_yield)
+slot_handle_level_4k(struct kvm *kvm, const struct kvm_memory_slot *memslot,
+		     slot_level_handler fn, bool flush_on_yield)
 {
 	return slot_handle_level(kvm, memslot, fn, PG_LEVEL_4K,
 				 PG_LEVEL_4K, flush_on_yield);
@@ -5821,7 +5821,12 @@ void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
 
 	if (kvm_memslots_have_rmaps(kvm)) {
 		write_lock(&kvm->mmu_lock);
-		flush = slot_handle_leaf(kvm, slot, kvm_mmu_zap_collapsible_spte, true);
+		/*
+		 * Zap only 4k SPTEs since the legacy MMU only supports dirty
+		 * logging at a 4k granularity and never creates collapsible
+		 * 2m SPTEs during dirty logging.
+		 */
+		flush = slot_handle_level_4k(kvm, slot, kvm_mmu_zap_collapsible_spte, true);
 		if (flush)
 			kvm_arch_flush_remote_tlbs_memslot(kvm, slot);
 		write_unlock(&kvm->mmu_lock);
@@ -5858,8 +5863,11 @@ void kvm_mmu_slot_leaf_clear_dirty(struct kvm *kvm,
 
 	if (kvm_memslots_have_rmaps(kvm)) {
 		write_lock(&kvm->mmu_lock);
-		flush = slot_handle_leaf(kvm, memslot, __rmap_clear_dirty,
-					 false);
+		/*
+		 * Clear dirty bits only on 4k SPTEs since the legacy MMU only
+		 * support dirty logging at a 4k granularity.
+		 */
+		flush = slot_handle_level_4k(kvm, memslot, __rmap_clear_dirty, false);
 		write_unlock(&kvm->mmu_lock);
 	}