1 files changed, 77 insertions, 32 deletions

diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
index d14ffc7513ee..02bc6b00d76c 100644
--- a/virt/kvm/dirty_ring.c
+++ b/virt/kvm/dirty_ring.c
@@ -55,9 +55,6 @@ static void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask)
 	struct kvm_memory_slot *memslot;
 	int as_id, id;
 
-	if (!mask)
-		return;
-
 	as_id = slot >> 16;
 	id = (u16)slot;
 
@@ -105,19 +102,38 @@ static inline bool kvm_dirty_gfn_harvested(struct kvm_dirty_gfn *gfn)
 	return smp_load_acquire(&gfn->flags) & KVM_DIRTY_GFN_F_RESET;
 }
 
-int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring)
+int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
+			 int *nr_entries_reset)
 {
+	/*
+	 * To minimize mmu_lock contention, batch resets for harvested entries
+	 * whose gfns are in the same slot, and are within N frame numbers of
+	 * each other, where N is the number of bits in an unsigned long.  For
+	 * simplicity, process the current set of entries when the next entry
+	 * can't be included in the batch.
+	 *
+	 * Track the current batch slot, the gfn offset into the slot for the
+	 * batch, and the bitmask of gfns that need to be reset (relative to
+	 * offset).  Note, the offset may be adjusted backwards, e.g. so that
+	 * a sequence of gfns X, X-1, ... X-N-1 can be batched.
+	 */
 	u32 cur_slot, next_slot;
 	u64 cur_offset, next_offset;
-	unsigned long mask;
-	int count = 0;
+	unsigned long mask = 0;
 	struct kvm_dirty_gfn *entry;
-	bool first_round = true;
 
-	/* This is only needed to make compilers happy */
-	cur_slot = cur_offset = mask = 0;
+	/*
+	 * Ensure concurrent calls to KVM_RESET_DIRTY_RINGS are serialized,
+	 * e.g. so that KVM fully resets all entries processed by a given call
+	 * before returning to userspace.  Holding slots_lock also protects
+	 * the various memslot accesses.
+	 */
+	lockdep_assert_held(&kvm->slots_lock);
+
+	while (likely((*nr_entries_reset) < INT_MAX)) {
+		if (signal_pending(current))
+			return -EINTR;
 
-	while (true) {
 		entry = &ring->dirty_gfns[ring->reset_index & (ring->size - 1)];
 
 		if (!kvm_dirty_gfn_harvested(entry))
@@ -130,35 +146,64 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring)
 		kvm_dirty_gfn_set_invalid(entry);
 
 		ring->reset_index++;
-		count++;
-		/*
-		 * Try to coalesce the reset operations when the guest is
-		 * scanning pages in the same slot.
-		 */
-		if (!first_round && next_slot == cur_slot) {
-			s64 delta = next_offset - cur_offset;
-
-			if (delta >= 0 && delta < BITS_PER_LONG) {
-				mask |= 1ull << delta;
-				continue;
+		(*nr_entries_reset)++;
+
+		if (mask) {
+			/*
+			 * While the size of each ring is fixed, it's possible
+			 * for the ring to be constantly re-dirtied/harvested
+			 * while the reset is in-progress (the hard limit exists
+			 * only to guard against the count becoming negative).
+			 */
+			cond_resched();
+
+			/*
+			 * Try to coalesce the reset operations when the guest
+			 * is scanning pages in the same slot.
+			 */
+			if (next_slot == cur_slot) {
+				s64 delta = next_offset - cur_offset;
+
+				if (delta >= 0 && delta < BITS_PER_LONG) {
+					mask |= 1ull << delta;
+					continue;
+				}
+
+				/* Backwards visit, careful about overflows! */
+				if (delta > -BITS_PER_LONG && delta < 0 &&
+				(mask << -delta >> -delta) == mask) {
+					cur_offset = next_offset;
+					mask = (mask << -delta) | 1;
+					continue;
+				}
 			}
 
-			/* Backwards visit, careful about overflows!  */
-			if (delta > -BITS_PER_LONG && delta < 0 &&
-			    (mask << -delta >> -delta) == mask) {
-				cur_offset = next_offset;
-				mask = (mask << -delta) | 1;
-				continue;
-			}
+			/*
+			 * Reset the slot for all the harvested entries that
+			 * have been gathered, but not yet fully processed.
+			 */
+			kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
 		}
-		kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
+
+		/*
+		 * The current slot was reset or this is the first harvested
+		 * entry, (re)initialize the metadata.
+		 */
 		cur_slot = next_slot;
 		cur_offset = next_offset;
 		mask = 1;
-		first_round = false;
 	}
 
-	kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
+	/*
+	 * Perform a final reset if there are harvested entries that haven't
+	 * been processed, which is guaranteed if at least one harvested was
+	 * found.  The loop only performs a reset when the "next" entry can't
+	 * be batched with the "current" entry(s), and that reset processes the
+	 * _current_ entry(s); i.e. the last harvested entry, a.k.a. next, will
+	 * always be left pending.
+	 */
+	if (mask)
+		kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
 
 	/*
 	 * The request KVM_REQ_DIRTY_RING_SOFT_FULL will be cleared
@@ -167,7 +212,7 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring)
 
 	trace_kvm_dirty_ring_reset(ring);
 
-	return count;
+	return 0;
 }
 
 void kvm_dirty_ring_push(struct kvm_vcpu *vcpu, u32 slot, u64 offset)