KVM: selftests: memslot_perf_test: Probe memory slots for once

prepare_vm() is called in every iteration and run. The allowed memory
slots (KVM_CAP_NR_MEMSLOTS) are probed for multiple times. It's not
free and unnecessary.

Move the probing logic for the allowed memory slots to parse_args()
for once, which is upper layer of prepare_vm().

No functional change intended.
Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221020071209.559062-4-gshan@redhat.com

tools/testing/selftests/kvm/memslot_perf_test.c

@@ -245,27 +245,17 @@ static bool prepare_vm(struct vm_data *data, int nslots, uint64_t *maxslots,
 		       void *guest_code, uint64_t mempages,
 		       struct timespec *slot_runtime)
 {
-	uint32_t max_mem_slots;
 	uint64_t rempages;
 	uint64_t guest_addr;
 	uint32_t slot;
 	struct timespec tstart;
 	struct sync_area *sync;
 
-	max_mem_slots = kvm_check_cap(KVM_CAP_NR_MEMSLOTS);
-	TEST_ASSERT(max_mem_slots > 1,
-		    "KVM_CAP_NR_MEMSLOTS should be greater than 1");
-	TEST_ASSERT(nslots > 1 || nslots == -1,
-		    "Slot count cap should be greater than 1");
-	if (nslots != -1)
-		max_mem_slots = min(max_mem_slots, (uint32_t)nslots);
-	pr_info_v("Allowed number of memory slots: %"PRIu32"\n", max_mem_slots);
-
 	TEST_ASSERT(mempages > 1,
 		    "Can't test without any memory");
 
 	data->npages = mempages;
-	data->nslots = max_mem_slots - 1;
+	data->nslots = nslots;
 	data->pages_per_slot = mempages / data->nslots;
 	if (!data->pages_per_slot) {
 		*maxslots = mempages + 1;
@@ -869,6 +859,7 @@ static void help(char *name, struct test_args *targs)
 static bool parse_args(int argc, char *argv[],
 		       struct test_args *targs)
 {
+	uint32_t max_mem_slots;
 	int opt;
 
 	while ((opt = getopt(argc, argv, "hvds:f:e:l:r:")) != -1) {
@@ -885,8 +876,8 @@ static bool parse_args(int argc, char *argv[],
 			break;
 		case 's':
 			targs->nslots = atoi(optarg);
-			if (targs->nslots <= 0 && targs->nslots != -1) {
-				pr_info("Slot count cap has to be positive or -1 for no cap\n");
+			if (targs->nslots <= 1 && targs->nslots != -1) {
+				pr_info("Slot count cap must be larger than 1 or -1 for no cap\n");
 				return false;
 			}
 			break;
@@ -932,6 +923,21 @@ static bool parse_args(int argc, char *argv[],
 		return false;
 	}
 
+	max_mem_slots = kvm_check_cap(KVM_CAP_NR_MEMSLOTS);
+	if (max_mem_slots <= 1) {
+		pr_info("KVM_CAP_NR_MEMSLOTS should be greater than 1\n");
+		return false;
+	}
+
+	/* Memory slot 0 is reserved */
+	if (targs->nslots == -1)
+		targs->nslots = max_mem_slots - 1;
+	else
+		targs->nslots = min_t(int, targs->nslots, max_mem_slots) - 1;
+
+	pr_info_v("Allowed Number of memory slots: %"PRIu32"\n",
+		  targs->nslots + 1);
+
 	return true;
 }