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

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Message-ID: <26E5DC9C-0F19-4E4F-9076-04506A197374@infradead.org>
Date:   Tue, 21 Feb 2023 10:27:31 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Oleksandr Natalenko <oleksandr@...alenko.name>
CC:     Kim Phillips <kim.phillips@....com>, tglx@...utronix.de,
        Usama Arif <usama.arif@...edance.com>, 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: [PATCH v9 0/8] Parallel CPU bringup for x86_64



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?

Hm, yes please. There's another one to make zero on the next line up, I think?

>> Or maybe we should just set up smpboot_control for the CPU to find its
>> own stuff, *even* on waking. Since the structures are already set up,
>> it isn't like a clean boot.
>> 
>> If you let it boot in parallel mode, what if you just *remove* the line
>> that sets smpboot_control=0 ?
>
>If the `smpboot_control = 0;` line in arch/x86/kernel/acpi/sleep.c::x86_acpi_suspend_lowlevel() is commented out, and the system is booted in parallel mode, then suspend/resume works.

Well that's entertaining. Now, can we come up with any theory which doesn't leave us wondering why it ever worked in the first place...?