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

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Date:   Wed, 24 Nov 2021 23:54:44 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "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>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to
 be brought up again.

On Mon, Nov 22 2021 at 16:47, Sebastian Andrzej Siewior wrote:
> 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.

Any comment from XEN folks?

Thanks,

        tglx