KVM: selftests: Tighten checks around prev iter's last dirty page in ring

Now that each iteration collects all dirty entries and ensures the guest
*completes* at least one write, tighten the exemptions for the last dirty
page of the previous iteration.  Specifically, the only legal value (other
than the current iteration) is N-1.

Unlike the last page for the current iteration, the in-progress write from
the previous iteration is guaranteed to have completed, otherwise the test
would have hung.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20250111003004.1235645-18-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/dirty_log_test.c

@@ -517,14 +517,22 @@ static void vm_dirty_log_verify(enum vm_guest_mode mode, unsigned long **bmap)
 
 			if (host_log_mode == LOG_MODE_DIRTY_RING) {
 				/*
-				 * The last page in the ring from this iteration
-				 * or the previous can be written with the value
-				 * from the previous iteration (relative to the
-				 * last page's iteration), as the value to be
-				 * written may be cached in a CPU register.
+				 * The last page in the ring from previous
+				 * iteration can be written with the value
+				 * from the previous iteration, as the value to
+				 * be written may be cached in a CPU register.
 				 */
-				if ((page == dirty_ring_last_page ||
-				     page == dirty_ring_prev_iteration_last_page) &&
+				if (page == dirty_ring_prev_iteration_last_page &&
+				    val == iteration - 1)
+					continue;
+
+				/*
+				 * Any value from a previous iteration is legal
+				 * for the last entry, as the write may not yet
+				 * have retired, i.e. the page may hold whatever
+				 * it had before this iteration started.
+				 */
+				if (page == dirty_ring_last_page &&
 				    val < iteration)
 					continue;
 			} else if (!val && iteration == 1 && bmap0_dirty) {