[xen staging] xen/cpupool: Don't allow removing cpu0 from cpupool0

[patchbot@xxxxxxx]

commit 544e8af41e23e0d1e787beaa512877f69e51fd76
Author:     Luca Fancellu <luca.fancellu@xxxxxxx>
AuthorDate: Fri May 6 13:00:10 2022 +0100
Commit:     Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
CommitDate: Mon May 9 13:32:39 2022 -0700

    xen/cpupool: Don't allow removing cpu0 from cpupool0
    
    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@xxxxxxx>
    Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
---
 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 86a175f99c..0a93bcc631 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);
--
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