linux-kernel - Re: [External] Re: [PATCH v9 0/8] Parallel CPU bringup for x86

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Message-ID: <049adfae-f4a0-349e-323a-52299969ef99@bytedance.com>
Date:   Tue, 21 Feb 2023 11:54:50 +0000
From:   Usama Arif <usama.arif@...edance.com>
To:     Oleksandr Natalenko <oleksandr@...alenko.name>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Kim Phillips <kim.phillips@....com>, tglx@...utronix.de,
        arjan@...ux.intel.com, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, x86@...nel.org,
        pbonzini@...hat.com, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        thomas.lendacky@....com, seanjc@...gle.com, pmenzel@...gen.mpg.de,
        fam.zheng@...edance.com, punit.agrawal@...edance.com,
        simon.evans@...edance.com, liangma@...ngbit.com,
        "Limonciello, Mario" <Mario.Limonciello@....com>,
        Piotr Gorski <piotrgorski@...hyos.org>
Subject: Re: [External] Re: [PATCH v9 0/8] Parallel CPU bringup for x86_64



On 21/02/2023 11:42, Oleksandr Natalenko wrote:
> On 21.02.2023 11:47, Usama Arif wrote:
>> On 21/02/2023 10:27, David Woodhouse wrote:
>>>
>>> On 21 February 2023 09:49:51 GMT, Oleksandr Natalenko 
>>> <oleksandr@...alenko.name> wrote:
>>>> On 21.02.2023 10:06, David Woodhouse wrote:
>>>>> Why does arch/x86/kernel/acpi/sleep.c::x86_acpi_suspend_lowlevel() set
>>>>>
>>>>>      initial_gs = per_cpu_offset(smp_processor_id()) ?
>>>>>
>>>>> Would it not be CPU#0 that comes back up, and should it not get
>>>>> per_cpu_offset(0) ?
>>>>
>>>> Wanna me try `initial_gs = per_cpu_offset(0);` too?
>>>
>>
>> I think it might be smp_processor_id() and not 0 incase CPU0 was 
>> offline at the point the system was suspended?
> 
> Is it even possible for CPU 0 to be offline, at least on x86?
> 

It is possible on x86 (using BOOTPARAM_HOTPLUG_CPU0), but I just read 
the Kconfig option and it says:

"resume from hibernate or suspend always starts from CPU0.
So hibernate and suspend are prevented if CPU0 is offline."

so I guess switching to 0 should be ok.