+++ This bug was initially created as a clone of Bug #636347 +++

Upstream commit d413e5d7 was applied after 0.8.3, but was backported to RHEL 6.0 as patch 185, so 0.8.1-26.el6 is also vulnerable.

Description of problem:
I just upgraded one of our servers to libvirt-0.8.4 but I can't use virsh or virt-manager anymore. Syslog says the following "error : get_cpu_value:88 : cannot open /sys/devices/system/cpu/cpu1/online: No such file or directory"
I only have /sys/devices/system/cpu/online which states the number of CPUs the server has, e.g. 0-7.

I suspected the kernel option "hotpluggable cpu support", which was not enabled on my host. After enabling it libvirt works again even though I still don't have "/sys/devices/system/cpu/cpu1/online".


I agree with danpb's analysis that commit d413e5d7 is the culprit.  I only tested that commit on a hot-plug enabled kernel.  So the fact that disabling hot-plugging makes /sys/devices/system/cpu/cpu<n>/ directories disappear makes sense, but was something I never encountered during my testing.

My patch only allowed a missing directory for cpu0 (since x86_64 systems with hot-unplug cpu support still disallow hot-unplugging cpu0).  The fix is to allow a missing directory for all possible cpus.





diff --git i/src/nodeinfo.c w/src/nodeinfo.c
index 65eeb24..3dac9f3 100644
--- i/src/nodeinfo.c
+++ w/src/nodeinfo.c
@@ -65,7 +65,8 @@ int linuxNodeInfoCPUPopulate(FILE *cpuinfo,
 /* Return the positive decimal contents of the given
  * CPU_SYS_PATH/cpu%u/FILE, or -1 on error.  If MISSING_OK and the
  * file could not be found, return 1 instead of an error; this is
- * because some machines cannot hot-unplug cpu0.  */
+ * because some machines cannot hot-unplug cpu0, or because
+ * hot-unplugging is disabled.  */
 static int
 get_cpu_value(unsigned int cpu, const char *file, bool missing_ok)
 {
@@ -113,7 +114,7 @@ cleanup:
 static int
 cpu_online(unsigned int cpu)
 {
-    return get_cpu_value(cpu, "online", cpu == 0);
+    return get_cpu_value(cpu, "online", true);
 }

 static unsigned long count_thread_siblings(unsigned int cpu)