qemu: Don't ignore CPU tuning config if required cgroups are missing

When domain XML contains any of the elements for setting up CPU
scheduling parameters (period, quota, emulator_period, or
emulator_quota) we need cpu cgroup to enforce the configuration.
However, the existing code would just ignore silently such settings if
either cgroups were not available at all cpu cgroup was not available.
Moreover, APIs for manipulating CPU scheduler parameters were already
failing if cpu cgroup was not available. This patch makes cpu cgroup
mandatory for all domains that use CPU scheduling elements in their XML.

src/qemu/qemu_cgroup.c

@@ -546,16 +546,21 @@ int qemuSetupCgroupForVcpu(struct qemud_driver *driver, virDomainObjPtr vm)
     unsigned long long period = vm->def->cputune.period;
     long long quota = vm->def->cputune.quota;
 
-    if (driver->cgroup == NULL)
-        return 0; /* Not supported, so claim success */
-
     if ((period || quota) &&
-        !qemuCgroupControllerActive(driver, VIR_CGROUP_CONTROLLER_CPU)) {
-        virReportError(VIR_ERR_SYSTEM_ERROR, "%s",
-                       _("cgroup cpu is not active"));
+        (!driver->cgroup ||
+         !qemuCgroupControllerActive(driver, VIR_CGROUP_CONTROLLER_CPU))) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                       _("cgroup cpu is required for scheduler tuning"));
         return -1;
     }
 
+    /* We are trying to setup cgroups for CPU pinning, which can also be done
+     * with virProcessInfoSetAffinity, thus the lack of cgroups is not fatal
+     * here.
+     */
+    if (driver->cgroup == NULL)
+        return 0;
+
     rc = virCgroupForDomain(driver->cgroup, vm->def->name, &cgroup, 0);
     if (rc != 0) {
         virReportSystemError(-rc,
@@ -636,6 +641,14 @@ int qemuSetupCgroupForEmulator(struct qemud_driver *driver,
     long long quota = vm->def->cputune.emulator_quota;
     int rc, i;
 
+    if ((period || quota) &&
+        (!driver->cgroup ||
+         !qemuCgroupControllerActive(driver, VIR_CGROUP_CONTROLLER_CPU))) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                       _("cgroup cpu is required for scheduler tuning"));
+        return -1;
+    }
+
     if (driver->cgroup == NULL)
         return 0; /* Not supported, so claim success */
 
@@ -656,10 +669,8 @@ int qemuSetupCgroupForEmulator(struct qemud_driver *driver,
     }
 
     for (i = 0; i < VIR_CGROUP_CONTROLLER_LAST; i++) {
-        if (!qemuCgroupControllerActive(driver, i)) {
-            VIR_WARN("cgroup %d is not active", i);
+        if (!qemuCgroupControllerActive(driver, i))
             continue;
-        }
         rc = virCgroupMoveTask(cgroup, cgroup_emulator, i);
         if (rc < 0) {
             virReportSystemError(-rc,