[PATCH v8 3/9] machine: Improve is_cpu_type_supported()

[Gavin Shan <gshan@redhat.com>]

It's no sense to check the CPU type when mc->valid_cpu_types[0] is
NULL. So the check is skipped for this particular case. The constraint
has been taken when the error messags are appended.

A precise hint for the error message is given when mc->valid_cpu_types[0]
is the only valid entry. Besides, enumeration on mc->valid_cpu_types[0]
when we have mutiple valid entries there is avoided to increase the code
readability, as suggested by Philippe Mathieu-Daudé.

Signed-off-by: Gavin Shan <gshan@redhat.com>
---
 hw/core/machine.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index 1797e002f9..c58c84abf5 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -1399,7 +1399,7 @@ static bool is_cpu_type_supported(const MachineState *machine, Error **errp)
      * CPU types have been determined. Note that the user specified CPU
      * type is provided through '-cpu' option.
      */
-    if (mc->valid_cpu_types && machine->cpu_type) {
+    if (machine->cpu_type && mc->valid_cpu_types && mc->valid_cpu_types[0]) {
         for (i = 0; mc->valid_cpu_types[i]; i++) {
             if (object_class_dynamic_cast(oc, mc->valid_cpu_types[i])) {
                 break;
@@ -1409,13 +1409,19 @@ static bool is_cpu_type_supported(const MachineState *machine, Error **errp)
         /* The user specified CPU type isn't valid */
         if (!mc->valid_cpu_types[i]) {
             error_setg(errp, "Invalid CPU type: %s", machine->cpu_type);
-            error_append_hint(errp, "The valid types are: %s",
-                              mc->valid_cpu_types[0]);
-            for (i = 1; mc->valid_cpu_types[i]; i++) {
-                error_append_hint(errp, ", %s", mc->valid_cpu_types[i]);
+            if (!mc->valid_cpu_types[1]) {
+                error_append_hint(errp, "The only valid type is: %s\n",
+                                  mc->valid_cpu_types[0]);
+            } else {
+                error_append_hint(errp, "The valid types are: ");
+                for (i = 0; mc->valid_cpu_types[i]; i++) {
+                    error_append_hint(errp, "%s%s",
+                                      mc->valid_cpu_types[i],
+                                      mc->valid_cpu_types[i + 1] ? ", " : "");
+                }
+                error_append_hint(errp, "\n");
             }
 
-            error_append_hint(errp, "\n");
             return false;
         }
     }
-- 
2.42.0