linux-kernel - [PATCH] cpu/hotplug: Allow the CPU in CPU_UP

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Message-ID: <20211122154714.xaoxok3fpk5bgznz@linutronix.de>
Date:   Mon, 22 Nov 2021 16:47:14 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>
Cc:     linux-kernel@...r.kernel.org,
        "Gonglei (Arei)" <arei.gonglei@...wei.com>, x86@...nel.org,
        xen-devel@...ts.xenproject.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be
 brought up again.

From: "Longpeng(Mike)" <longpeng2@...wei.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@...wei.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Link: https://lore.kernel.org/r/20210901051143.2752-1-longpeng2@huawei.com
---

For XEN: this changes the behaviour as it allows to invoke
cpu_initialize_context() again should it have have earlier. I *think*
this is okay and would to bring up the CPU again should the memory
allocation in cpu_initialize_context() fail.

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

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index f6bc0bc8a2aab..34958d7fe2c1c 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. */
-- 
2.33.1