qemu: vcpu: Clear vcpu order information rather than making it invalid

Certain operations may make the vcpu order information invalid. Since
the order is primarily used to ensure migration compatibility and has
basically no other user benefits, clear the order prior to certain
operations and document that it may be cleared.

All the operations that would clear the order can still be properly
executed by defining a new domain configuration rather than using the
helper APIs.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1370357

docs/formatdomain.html.in

@@ -569,7 +569,8 @@
         the order may be be duplicated accross all vcpus that need to be
         enabled at once. Specifying order is not necessary, vcpus are then
         added in an arbitrary order. If order info is used, it must be used for
-        all online vcpus.
+        all online vcpus. Hypervisors may clear or update ordering information
+        during certain operations to assure valid configuration.
 
         Note that hypervisors may create hotpluggable vcpus differently from
         boot vcpus thus special initialization may be necessary.

src/conf/domain_conf.c

@@ -25566,3 +25566,13 @@ virDomainGetBlkioParametersAssignFromDef(virDomainDefPtr def,
     virBufferFreeAndReset(&buf);
     return -1;
 }
+
+
+void
+virDomainDefVcpuOrderClear(virDomainDefPtr def)
+{
+    size_t i;
+
+    for (i = 0; i < def->maxvcpus; i++)
+        def->vcpus[i]->order = 0;
+}

src/conf/domain_conf.h

@@ -2487,6 +2487,7 @@ unsigned int virDomainDefGetVcpus(const virDomainDef *def);
 virBitmapPtr virDomainDefGetOnlineVcpumap(const virDomainDef *def);
 virDomainVcpuDefPtr virDomainDefGetVcpu(virDomainDefPtr def, unsigned int vcpu)
     ATTRIBUTE_RETURN_CHECK;
+void virDomainDefVcpuOrderClear(virDomainDefPtr def);
 
 virDomainObjPtr virDomainObjNew(virDomainXMLOptionPtr caps)
     ATTRIBUTE_NONNULL(1);

src/libvirt_private.syms

@@ -264,6 +264,7 @@ virDomainDefSetMemoryTotal;
 virDomainDefSetVcpus;
 virDomainDefSetVcpusMax;
 virDomainDefValidate;
+virDomainDefVcpuOrderClear;
 virDomainDeleteConfig;
 virDomainDeviceAddressIsValid;
 virDomainDeviceAddressTypeToString;

src/qemu/qemu_driver.c

@@ -4745,6 +4745,9 @@ qemuDomainSetVcpusMax(virQEMUDriverPtr driver,
         }
     }
 
+    /* ordering information may become invalid, thus clear it */
+    virDomainDefVcpuOrderClear(persistentDef);
+
     if (virDomainDefSetVcpusMax(persistentDef, nvcpus, driver->xmlopt) < 0)
         goto cleanup;
 
@@ -4919,6 +4922,8 @@ qemuDomainSetVcpusConfig(virDomainDefPtr def,
     size_t maxvcpus = virDomainDefGetVcpusMax(def);
     size_t i;
 
+    /* ordering information may become invalid, thus clear it */
+    virDomainDefVcpuOrderClear(def);
 
     if (curvcpus == nvcpus)
         return;