qemu: Forbid config when topology based cpu count doesn't match the config

As of qemu commit:
commit a32ef3bfc12c8d0588f43f74dcc5280885bbdb30
Author: Thomas Huth <thuth@redhat.com>
Date:   Wed Jul 22 15:59:50 2015 +0200

    vl: Add another sanity check to smp_parse() function

v2.4.0-952-ga32ef3b

configuration where the maximum CPU count doesn't match the topology is
rejected. Prior to that only configurations where the topology would
contain more cpus than the maximum count would be rejected.

Use QEMU_CAPS_QUERY_HOTPLUGGABLE_CPUS as a relevant recent enough
witness to avoid breaking old configs.

src/qemu/qemu_domain.c

@@ -2314,12 +2314,21 @@ qemuDomainDefPostParse(virDomainDefPtr def,
 static int
 qemuDomainDefValidate(const virDomainDef *def,
                       virCapsPtr caps ATTRIBUTE_UNUSED,
-                      void *opaque ATTRIBUTE_UNUSED)
+                      void *opaque)
 {
+    virQEMUDriverPtr driver = opaque;
+    virQEMUCapsPtr qemuCaps = NULL;
+    size_t topologycpus;
+    int ret = -1;
+
+    if (!(qemuCaps = virQEMUCapsCacheLookup(driver->qemuCapsCache,
+                                            def->emulator)))
+        goto cleanup;
+
     if (def->mem.min_guarantee) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                        _("Parameter 'min_guarantee' not supported by QEMU."));
-        return -1;
+        goto cleanup;
     }
 
     if (def->os.loader &&
@@ -2330,7 +2339,7 @@ qemuDomainDefValidate(const virDomainDef *def,
         if (!qemuDomainMachineIsQ35(def)) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                            _("Secure boot is supported with q35 machine types only"));
-            return -1;
+            goto cleanup;
         }
 
         /* Now, technically it is possible to have secure boot on
@@ -2339,17 +2348,34 @@ qemuDomainDefValidate(const virDomainDef *def,
         if (def->os.arch != VIR_ARCH_X86_64) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                            _("Secure boot is supported for x86_64 architecture only"));
-            return -1;
+            goto cleanup;
         }
 
         if (def->features[VIR_DOMAIN_FEATURE_SMM] != VIR_TRISTATE_SWITCH_ON) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                            _("Secure boot requires SMM feature enabled"));
-            return -1;
+            goto cleanup;
         }
     }
 
-    return 0;
+    /* qemu as of 2.5.0 rejects SMP topologies that don't match the cpu count */
+    if (def->cpu && def->cpu->sockets) {
+        topologycpus = def->cpu->sockets * def->cpu->cores * def->cpu->threads;
+        if (topologycpus != virDomainDefGetVcpusMax(def)) {
+            /* presence of query-hotpluggable-cpus should be a good enough witness */
+            if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_QUERY_HOTPLUGGABLE_CPUS)) {
+                virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                               _("CPU topology doesn't match maximum vcpu count"));
+                goto cleanup;
+            }
+        }
+    }
+
+    ret = 0;
+
+ cleanup:
+    virObjectUnref(qemuCaps);
+    return ret;
 }