linux-kernel - [PATCH] KVM: Eliminate extra function calls in kvm_get_dirty_log

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Message-Id: <20150317161958.991bd7dc36c714f6498821db@lab.ntt.co.jp>
Date:	Tue, 17 Mar 2015 16:19:58 +0900
From:	Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>
To:	pbonzini@...hat.com
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] KVM: Eliminate extra function calls in
 kvm_get_dirty_log_protect()

When all bits in mask are not set,
kvm_arch_mmu_enable_log_dirty_pt_masked() has nothing to do.  But since
it needs to be called from the generic code, it cannot be inlined, and
a few function calls, two when PML is enabled, are wasted.

Since it is common to see many pages remain clean, e.g. framebuffers can
stay calm for a long time, it is worth eliminating this overhead.

Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>
---
 virt/kvm/kvm_main.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index a109370..420d8cf 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1061,9 +1061,11 @@ int kvm_get_dirty_log_protect(struct kvm *kvm,
 		mask = xchg(&dirty_bitmap[i], 0);
 		dirty_bitmap_buffer[i] = mask;
 
-		offset = i * BITS_PER_LONG;
-		kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot, offset,
-								mask);
+		if (mask) {
+			offset = i * BITS_PER_LONG;
+			kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
+								offset, mask);
+		}
 	}
 
 	spin_unlock(&kvm->mmu_lock);
-- 
2.1.0

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