[PATCH 07/20] KVM: Avoid useless memory write when possible

[Avi Kivity]

From: Luca Tettamanti <kronos.it@xxxxxxxxx>

When writing to normal memory and the memory area is unchanged the write
can be safely skipped, avoiding the costly kvm_mmu_pte_write.

Signed-Off-By: Luca Tettamanti <kronos.it@xxxxxxxxx>
Signed-off-by: Avi Kivity <avi@xxxxxxxxxxxx>
---
 drivers/kvm/kvm_main.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
index 7826f16..5603000 100644
--- a/drivers/kvm/kvm_main.c
+++ b/drivers/kvm/kvm_main.c
@@ -1170,8 +1170,10 @@ static int emulator_write_phys(struct kvm_vcpu *vcpu, gpa_t gpa,
 		return 0;
 	mark_page_dirty(vcpu->kvm, gpa >> PAGE_SHIFT);
 	virt = kmap_atomic(page, KM_USER0);
-	kvm_mmu_pte_write(vcpu, gpa, virt + offset, val, bytes);
-	memcpy(virt + offset_in_page(gpa), val, bytes);
+	if (memcmp(virt + offset_in_page(gpa), val, bytes)) {
+		kvm_mmu_pte_write(vcpu, gpa, virt + offset, val, bytes);
+		memcpy(virt + offset_in_page(gpa), val, bytes);
+	}
 	kunmap_atomic(virt, KM_USER0);
 	return 1;
 }
-- 
1.5.2.2

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