s390/mm: KVM: pv: when tearing down, try to destroy protected pages

When ptep_get_and_clear_full is called for a mm teardown, we will now
attempt to destroy the secure pages. This will be faster than export.

In case it was not a teardown, or if for some reason the destroy page
UVC failed, we try with an export page, like before.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Link: https://lore.kernel.org/r/20220628135619.32410-11-imbrenda@linux.ibm.com
Message-Id: <20220628135619.32410-11-imbrenda@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>

arch/s390/include/asm/pgtable.h

@@ -1182,9 +1182,22 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
 	} else {
 		res = ptep_xchg_lazy(mm, addr, ptep, __pte(_PAGE_INVALID));
 	}
-	/* At this point the reference through the mapping is still present */
-	if (mm_is_protected(mm) && pte_present(res))
-		uv_convert_owned_from_secure(pte_val(res) & PAGE_MASK);
+	/* Nothing to do */
+	if (!mm_is_protected(mm) || !pte_present(res))
+		return res;
+	/*
+	 * At this point the reference through the mapping is still present.
+	 * The notifier should have destroyed all protected vCPUs at this
+	 * point, so the destroy should be successful.
+	 */
+	if (full && !uv_destroy_owned_page(pte_val(res) & PAGE_MASK))
+		return res;
+	/*
+	 * If something went wrong and the page could not be destroyed, or
+	 * if this is not a mm teardown, the slower export is used as
+	 * fallback instead.
+	 */
+	uv_convert_owned_from_secure(pte_val(res) & PAGE_MASK);
 	return res;
 }