xen/percpu: don't initialize percpu on resume

Invocation of the CPU_UP_PREPARE notification
on ARM64 during resume causes a crash:

(XEN) [  315.807606] Error bringing CPU1 up: -16
(XEN) [  315.811926] Xen BUG at common/cpu.c:258
[...]
(XEN) [  316.142765] Xen call trace:
(XEN) [  316.146048]    [<00000a0000202264>] enable_nonboot_cpus+0x128/0x1ac (PC)
(XEN) [  316.153219]    [<00000a000020225c>] enable_nonboot_cpus+0x120/0x1ac (LR)
(XEN) [  316.160391]    [<00000a0000278180>] suspend.c#system_suspend+0x4c/0x1a0
(XEN) [  316.167476]    [<00000a0000206b70>] domain.c#continue_hypercall_tasklet_handler+0x54/0xd0
(XEN) [  316.176117]    [<00000a0000226538>] tasklet.c#do_tasklet_work+0xb8/0x100
(XEN) [  316.183288]    [<00000a0000226920>] do_tasklet+0x68/0xb0
(XEN) [  316.189077]    [<00000a000026e120>] domain.c#idle_loop+0x7c/0x194
(XEN) [  316.195644]    [<00000a0000277638>] shutdown.c#halt_this_cpu+0/0x14
(XEN) [  316.202383]    [<0000000000000008>] 0000000000000008

Freeing per-CPU areas and setting __per_cpu_offset to INVALID_PERCPU_AREA
only occur when !park_offline_cpus and system_state is not SYS_STATE_suspend.
On ARM64, park_offline_cpus is always false, so setting __per_cpu_offset to
INVALID_PERCPU_AREA depends solely on the system state.

If the system is suspended, this area is not freed, and during resume, an error
occurs in init_percpu_area, causing a crash because INVALID_PERCPU_AREA is not
set and park_offline_cpus remains 0:

    if ( __per_cpu_offset[cpu] != INVALID_PERCPU_AREA )
        return park_offline_cpus ? 0 : -EBUSY;

The same crash can occur on x86 if park_offline_cpus is set
to 0 during Xen resume.

Fixes: f75780d26b ("xen: move per-cpu area management into common code")
Signed-off-by: Mykyta Poturai <mykyta_poturai@epam.com>
Signed-off-by: Mykola Kvach <mykola_kvach@epam.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/common/percpu.c

@@ -30,7 +30,9 @@ static int init_percpu_area(unsigned int cpu)
     char *p;
 
     if ( __per_cpu_offset[cpu] != INVALID_PERCPU_AREA )
-        return park_offline_cpus ? 0 : -EBUSY;
+        return park_offline_cpus || system_state == SYS_STATE_resume
+               ? 0
+               : -EBUSY;
 
     if ( (p = alloc_xenheap_pages(PERCPU_ORDER, 0)) == NULL )
         return -ENOMEM;