Revert "KVM: Avoid useless memory write when possible"

This reverts commit a3c870bd.  While it
does save useless updates, it (probably) defeats the fork detector, causing
a massive performance loss.
Signed-off-by: Avi Kivity <avi@qumranet.com>

drivers/kvm/kvm_main.c

@@ -1070,10 +1070,8 @@ 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;
 }