[PATCH 3/3] KVM: MMU: Separate trivial NULL check out from rmap_get_next()

[Takuya Yoshikawa <yoshikawa.takuya@xxxxxxxxxxxxx>]

Checking wheter iter->desc is NULL is not worth a function call
especially when we use EPT/NPT because we know it would almost always
be NULL.

Although using "inline" like this does not look clean, we could see
measurable performance improvements: get_dirty_log for 1GB dirty memory
became faster by more than 10% on my test box.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@xxxxxxxxxxxxx>
---
 arch/x86/kvm/mmu.c |   38 +++++++++++++++++++++++---------------
 1 files changed, 23 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index d042087..8aa58a1 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1016,30 +1016,38 @@ static bool rmap_get_first(unsigned long rmap, struct rmap_iterator *iter)
  *
  * Returns true if spte is found, false otherwise.
  */
-static bool rmap_get_next(struct rmap_iterator *iter)
+static bool __rmap_get_next(struct rmap_iterator *iter)
 {
-	if (iter->desc) {
-		if (iter->pos < PTE_LIST_EXT - 1) {
-			++iter->pos;
-			iter->sptep = iter->desc->sptes[iter->pos];
-			if (iter->sptep)
-				return true;
-		}
+	if (iter->pos < PTE_LIST_EXT - 1) {
+		++iter->pos;
+		iter->sptep = iter->desc->sptes[iter->pos];
+		if (iter->sptep)
+			return true;
+	}
 
-		iter->desc = iter->desc->more;
+	iter->desc = iter->desc->more;
 
-		if (iter->desc) {
-			iter->pos = 0;
-			iter->sptep = iter->desc->sptes[iter->pos];
-			/* desc->sptes[0] cannot be NULL */
-			return true;
-		}
+	if (iter->desc) {
+		iter->pos = 0;
+		iter->sptep = iter->desc->sptes[iter->pos];
+		/* desc->sptes[0] cannot be NULL */
+		return true;
 	}
 
 	iter->sptep = NULL;
 	return false;
 }
 
+/* Make this trivial check alone inlined by putting inline. */
+static inline bool rmap_get_next(struct rmap_iterator *iter)
+{
+	if (iter->desc)
+		return __rmap_get_next(iter);
+
+	iter->sptep = NULL;
+	return false;
+}
+
 static void drop_spte(struct kvm *kvm, u64 *sptep)
 {
 	if (mmu_spte_clear_track_bits(sptep))
-- 
1.7.5.4

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