KVM: Avoid useless memory write when possible

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@gmail.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
index 7826f16271e5..5603000573ec 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;
 }