arm64: mm: Add asid_gen_match() helper

Add a macro to check if an ASID is from the current generation, since a
subsequent patch will introduce a third user for this test.
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/r/20200519175502.2504091-6-jean-philippe@linaro.orgSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/mm/context.c

@@ -92,6 +92,9 @@ static void set_reserved_asid_bits(void)
 		bitmap_clear(asid_map, 0, NUM_USER_ASIDS);
 }
 
+#define asid_gen_match(asid) \
+	(!(((asid) ^ atomic64_read(&asid_generation)) >> asid_bits))
+
 static void flush_context(void)
 {
 	int i;
@@ -220,8 +223,7 @@ void check_and_switch_context(struct mm_struct *mm, unsigned int cpu)
 	 *   because atomic RmWs are totally ordered for a given location.
 	 */
 	old_active_asid = atomic64_read(&per_cpu(active_asids, cpu));
-	if (old_active_asid &&
-	    !((asid ^ atomic64_read(&asid_generation)) >> asid_bits) &&
+	if (old_active_asid && asid_gen_match(asid) &&
 	    atomic64_cmpxchg_relaxed(&per_cpu(active_asids, cpu),
 				     old_active_asid, asid))
 		goto switch_mm_fastpath;
@@ -229,7 +231,7 @@ void check_and_switch_context(struct mm_struct *mm, unsigned int cpu)
 	raw_spin_lock_irqsave(&cpu_asid_lock, flags);
 	/* Check that our ASID belongs to the current generation. */
 	asid = atomic64_read(&mm->context.id);
-	if ((asid ^ atomic64_read(&asid_generation)) >> asid_bits) {
+	if (!asid_gen_match(asid)) {
 		asid = new_context(mm);
 		atomic64_set(&mm->context.id, asid);
 	}