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

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Date:   Wed, 24 Nov 2021 05:46:23 +0000
From:   Henry Wang <Henry.Wang@....com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Gonglei (Arei)" <arei.gonglei@...wei.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "xen-devel@...ts.xenproject.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: RE: [PATCH] cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be
 brought up again.

Hi,

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@...ts.xenproject.org> On Behalf Of
> Sebastian Andrzej Siewior
> Sent: Monday, November 22, 2021 11:47 PM
> 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@...wei.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
> 

Reviewed-by: Henry Wang <Henry.Wang@....com>

Kind regards,

Henry