[RFC PATCH 1/5] KVM: x86/mmu: Walk memslot rmaps by index

[Fred Griffoul <griffoul@gmail.com>]

From: Fred Griffoul <fgriffo@amazon.co.uk>

In preparation for storing memslot rmaps in chunks that are not
contiguous in memory, walk a slot's rmaps by index instead of stepping
a pointer through the array, and look each head up through a new
__idx_to_rmap() helper.

No functional change intended.

Signed-off-by: Fred Griffoul <fgriffo@amazon.co.uk>
---
 arch/x86/kvm/mmu/mmu.c | 34 ++++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index f4b12d6aef55..b3fcd6f2a991 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -1236,13 +1236,17 @@ unsigned int pte_list_count(struct kvm_rmap_head *rmap_head)
 	return desc->tail_count + desc->spte_count;
 }
 
+static struct kvm_rmap_head *__idx_to_rmap(const struct kvm_memory_slot *slot,
+					   int level, unsigned long idx)
+{
+	return &slot->arch.rmap[level - PG_LEVEL_4K][idx];
+}
+
 static struct kvm_rmap_head *gfn_to_rmap(gfn_t gfn, int level,
 					 const struct kvm_memory_slot *slot)
 {
-	unsigned long idx;
-
-	idx = gfn_to_index(gfn, slot->base_gfn, level);
-	return &slot->arch.rmap[level - PG_LEVEL_4K][idx];
+	return __idx_to_rmap(slot, level,
+			     gfn_to_index(gfn, slot->base_gfn, level));
 }
 
 static void rmap_remove(struct kvm *kvm, u64 *spte)
@@ -1577,17 +1581,21 @@ struct slot_rmap_walk_iterator {
 	struct kvm_rmap_head *rmap;
 	int level;
 
-	/* private field. */
-	struct kvm_rmap_head *end_rmap;
+	/* private fields. */
+	unsigned long idx;
+	unsigned long end_idx;
 };
 
 static void rmap_walk_init_level(struct slot_rmap_walk_iterator *iterator,
 				 int level)
 {
+	const struct kvm_memory_slot *slot = iterator->slot;
+
 	iterator->level = level;
 	iterator->gfn = iterator->start_gfn;
-	iterator->rmap = gfn_to_rmap(iterator->gfn, level, iterator->slot);
-	iterator->end_rmap = gfn_to_rmap(iterator->end_gfn, level, iterator->slot);
+	iterator->idx = gfn_to_index(iterator->start_gfn, slot->base_gfn, level);
+	iterator->end_idx = gfn_to_index(iterator->end_gfn, slot->base_gfn, level);
+	iterator->rmap = gfn_to_rmap(iterator->gfn, level, slot);
 }
 
 static void slot_rmap_walk_init(struct slot_rmap_walk_iterator *iterator,
@@ -1611,11 +1619,17 @@ static bool slot_rmap_walk_okay(struct slot_rmap_walk_iterator *iterator)
 
 static void slot_rmap_walk_next(struct slot_rmap_walk_iterator *iterator)
 {
-	while (++iterator->rmap <= iterator->end_rmap) {
+	struct kvm_rmap_head *rmap_head;
+
+	while (++iterator->idx <= iterator->end_idx) {
 		iterator->gfn += KVM_PAGES_PER_HPAGE(iterator->level);
 
-		if (atomic_long_read(&iterator->rmap->val))
+		rmap_head = __idx_to_rmap(iterator->slot, iterator->level,
+					  iterator->idx);
+		if (atomic_long_read(&rmap_head->val)) {
+			iterator->rmap = rmap_head;
 			return;
+		}
 	}
 
 	if (++iterator->level > iterator->end_level) {
-- 
2.47.3