linux-kernel - [PATCH] KVM: x86/mmu: Speed up slot_rmap_walk

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Date:   Fri, 25 Mar 2022 23:31:25 +0000
From:   Vipin Sharma <vipinsh@...gle.com>
To:     pbonzini@...hat.com
Cc:     seanjc@...gle.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, dmatlack@...gle.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vipin Sharma <vipinsh@...gle.com>
Subject: [PATCH] KVM: x86/mmu: Speed up slot_rmap_walk_next for sparsely
 populated rmaps

Avoid calling handlers on empty rmap entries and skip to the next non
empty rmap entry.

Empty rmap entries are noop in handlers.

Signed-off-by: Vipin Sharma <vipinsh@...gle.com>
Suggested-by: Sean Christopherson <seanjc@...gle.com>
Change-Id: I8abf0f4d82a2aae4c5d58b80bcc17ffc30785ffc
---
 arch/x86/kvm/mmu/mmu.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 51671cb34fb6..f296340803ba 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -1499,11 +1499,14 @@ static bool slot_rmap_walk_okay(struct slot_rmap_walk_iterator *iterator)
 	return !!iterator->rmap;
 }
 
-static void slot_rmap_walk_next(struct slot_rmap_walk_iterator *iterator)
+static noinline void
+slot_rmap_walk_next(struct slot_rmap_walk_iterator *iterator)
 {
-	if (++iterator->rmap <= iterator->end_rmap) {
+	while (++iterator->rmap <= iterator->end_rmap) {
 		iterator->gfn += (1UL << KVM_HPAGE_GFN_SHIFT(iterator->level));
-		return;
+
+		if (iterator->rmap->val)
+			return;
 	}
 
 	if (++iterator->level > iterator->end_level) {

base-commit: c9b8fecddb5bb4b67e351bbaeaa648a6f7456912
-- 
2.35.1.1021.g381101b075-goog