Cpu0 must remain in cpupool0, otherwise some operations like moving cpus
between cpupools, cpu hotplug, destroying cpupools, shutdown of the host,
might not work in a sane way.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
---
Changes in v8:
- Add R-by (Juergen)
Changes in v7:
- new patch
---
 xen/common/sched/cpupool.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/xen/common/sched/cpupool.c b/xen/common/sched/cpupool.c
index 86a175f99cd5..0a93bcc631bf 100644
--- a/xen/common/sched/cpupool.c
+++ b/xen/common/sched/cpupool.c
@@ -572,6 +572,7 @@ static long cf_check cpupool_unassign_cpu_helper(void *info)
  * possible failures:
  * - last cpu and still active domains in cpupool
  * - cpu just being unplugged
+ * - Attempt to remove boot cpu from cpupool0
  */
 static int cpupool_unassign_cpu(struct cpupool *c, unsigned int cpu)
 {
@@ -582,7 +583,12 @@ static int cpupool_unassign_cpu(struct cpupool *c, unsigned int cpu)
     debugtrace_printk("cpupool_unassign_cpu(pool=%u,cpu=%d)\n",
                       c->cpupool_id, cpu);
 
-    if ( !cpu_online(cpu) )
+    /*
+     * Cpu0 must remain in cpupool0, otherwise some operations like moving cpus
+     * between cpupools, cpu hotplug, destroying cpupools, shutdown of the host,
+     * might not work in a sane way.
+     */
+    if ( (!c->cpupool_id && !cpu) || !cpu_online(cpu) )
         return -EINVAL;
 
     master_cpu = sched_get_resource_cpu(cpu);
-- 
2.17.1