On Mon, Jan 25, 2021 at 07:27:09PM +0100, Borislav Petkov wrote:

quoted

+		pte_t old_pte, new_pte;
+
+		do {
+			old_pte = READ_ONCE(*ptep);
+			new_pte = pte_wrprotect(old_pte);

Maybe I'm missing something but those two can happen outside of the
loop, no? Or is *ptep somehow changing concurrently while the loop is
doing the CMPXCHG and you need to recreate it each time?

IOW, you can generate upfront and do the empty loop...

quoted

+
+		} while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));
+
+		return;
+	}

Empty loop would be wrong, but that wants to be written like:

	old_pte = READ_ONCE(*ptep);
	do {
		new_pte = pte_wrprotect(old_pte);
	} while (try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));

Since try_cmpxchg() will update old_pte on failure.