qemu: Enforce vCPU hotplug granularity constraints

QEMU 2.7 and newer don't allow guests to start unless the initial
vCPUs count is a multiple of the vCPU hotplug granularity, so
validate it and report an error if needed.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1283700Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

src/qemu/qemu_domain.c

@@ -3289,6 +3289,36 @@ qemuDomainDefValidateVideo(const virDomainDef *def)
 }
 
 
+/**
+ * qemuDomainDefGetVcpuHotplugGranularity:
+ * @def: domain definition
+ *
+ * With QEMU 2.7 and newer, vCPUs can only be hotplugged in groups that
+ * respect the guest's hotplug granularity; because of that, QEMU will
+ * not allow guests to start unless the initial number of vCPUs is a
+ * multiple of the hotplug granularity.
+ *
+ * Returns the vCPU hotplug granularity.
+ */
+static unsigned int
+qemuDomainDefGetVcpuHotplugGranularity(const virDomainDef *def)
+{
+    /* If the guest CPU topology has not been configured, assume we
+     * can hotplug vCPUs one at a time */
+    if (!def->cpu || def->cpu->sockets == 0)
+        return 1;
+
+    /* For pSeries guests, hotplug can only be performed one core
+     * at a time, so the vCPU hotplug granularity is the number
+     * of threads per core */
+    if (qemuDomainIsPSeries(def))
+        return def->cpu->threads;
+
+    /* In all other cases, we can hotplug vCPUs one at a time */
+    return 1;
+}
+
+
 #define QEMU_MAX_VCPUS_WITHOUT_EIM 255
 
 
@@ -3363,6 +3393,7 @@ qemuDomainDefValidate(const virDomainDef *def,
      * CPU topology. Verify known constraints are respected */
     if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_QUERY_HOTPLUGGABLE_CPUS)) {
         unsigned int topologycpus;
+        unsigned int granularity;
 
         /* Starting from QEMU 2.5, max vCPU count and overall vCPU topology
          * must agree. We only actually enforce this with QEMU 2.7+, due
@@ -3373,6 +3404,16 @@ qemuDomainDefValidate(const virDomainDef *def,
                            _("CPU topology doesn't match maximum vcpu count"));
             goto cleanup;
         }
+
+        /* vCPU hotplug granularity must be respected */
+        granularity = qemuDomainDefGetVcpuHotplugGranularity(def);
+        if ((virDomainDefGetVcpus(def) % granularity) != 0) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                           _("vCPUs count must be a multiple of the vCPU "
+                             "hotplug granularity (%u)"),
+                           granularity);
+            goto cleanup;
+        }
     }
 
     if (ARCH_IS_X86(def->os.arch) &&

tests/qemuxml2argvdata/cpu-hotplug-granularity.xml

+<domain type='qemu'>
+  <name>guest</name>
+  <uuid>1ccfd97d-5eb4-478a-bbe6-88d254c16db7</uuid>
+  <memory unit='KiB'>524288</memory>
+  <vcpu placement='static' current='2'>8</vcpu>
+  <os>
+    <type arch='ppc64' machine='pseries'>hvm</type>
+  </os>
+  <cpu>
+    <topology sockets='1' cores='2' threads='4'/>
+  </cpu>
+  <devices>
+    <emulator>/usr/bin/qemu-system-ppc64</emulator>
+    <controller type='pci' model='pci-root'/>
+    <controller type='usb' model='none'/>
+    <memballoon model='none'/>
+  </devices>
+</domain>

tests/qemuxml2argvtest.c

@@ -2900,6 +2900,9 @@ mymain(void)
             QEMU_CAPS_DEVICE_INTEL_IOMMU);
 
     DO_TEST("cpu-hotplug-startup", QEMU_CAPS_QUERY_HOTPLUGGABLE_CPUS);
+    DO_TEST_PARSE_ERROR("cpu-hotplug-granularity",
+                        QEMU_CAPS_QUERY_HOTPLUGGABLE_CPUS);
+
     DO_TEST("virtio-options", QEMU_CAPS_VIRTIO_SCSI, QEMU_CAPS_VIRTIO_KEYBOARD,
             QEMU_CAPS_VIRTIO_MOUSE, QEMU_CAPS_VIRTIO_TABLET,
             QEMU_CAPS_VIRTIO_INPUT_HOST,