[tip: smp/core] cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be brought up again.

[tip-bot2 for Longpeng(Mike)]

The following commit has been merged into the smp/core branch of tip:

Commit-ID:     c7dfb2591b23856182e2c9da6b1d526ef7fd6b2e
Gitweb:        https://git.kernel.org/tip/c7dfb2591b23856182e2c9da6b1d526ef7fd6b2e
Author:        Longpeng(Mike) <longpeng2@xxxxxxxxxx>
AuthorDate:    Wed, 09 Feb 2022 09:02:14 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 12 Apr 2022 14:13:01 +02:00

cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be brought up again.

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@xxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Reviewed-by: Henry Wang <Henry.Wang@xxxxxxx>
Link: https://lore.kernel.org/r/20220209080214.1439408-3-bigeasy@xxxxxxxxxxxxx

---
 kernel/smpboot.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index f6bc0bc..b9f5454 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. */