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Subject[PATCH 2/2] cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be brought up again.
Date
From: "Longpeng(Mike)" <longpeng2@huawei.com>

A CPU will not show up in virtualized environment which includes an
Enclave. The VM splits its resources into a primary VM and a Enclave
VM. While the Enclave is active, the hypervisor will ignore all requests
to bring up a CPU and this CPU will remain in CPU_UP_PREPARE state.
The kernel will wait up to ten seconds for CPU to show up
(do_boot_cpu()) and then rollback the hotplug state back to
CPUHP_OFFLINE leaving the CPU state in CPU_UP_PREPARE. The CPU state is
set back to CPUHP_TEARDOWN_CPU during the CPU_POST_DEAD stage.

After the Enclave VM terminates, the primary VM can bring up the CPU
again.

Allow to bring up the CPU if it is in the CPU_UP_PREPARE state.

[bigeasy: Rewrite commit description.]

Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20210901051143.2752-1-longpeng2@huawei.com
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>
Link: https://lore.kernel.org/r/20211122154714.xaoxok3fpk5bgznz@linutronix.de
---
kernel/smpboot.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index f6bc0bc8a2aab..b9f54544e7499 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -392,6 +392,13 @@ int cpu_check_up_prepare(int cpu)
*/
return -EAGAIN;

+ case CPU_UP_PREPARE:
+ /*
+ * Timeout while waiting for the CPU to show up. Allow to try
+ * again later.
+ */
+ return 0;
+
default:

/* Should not happen. Famous last words. */
--
2.34.1
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