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: NLuca Tettamanti <kronos.it@gmail.com>
Signed-off-by: NAvi Kivity <avi@qumranet.com>

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);
+	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;
 }