linux-kernel - Re: [PATCH v12 06/11] x86/smpboot: Remove initial

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Date:   Tue, 28 Feb 2023 17:22:46 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Usama Arif <usama.arif@...edance.com>, dwmw2@...radead.org,
        kim.phillips@....com, piotrgorski@...hyos.org,
        oleksandr@...alenko.name, 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,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v12 06/11] x86/smpboot: Remove initial_stack on 64-bit

On Tue, Feb 28, 2023 at 11:13 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Sun, Feb 26 2023 at 11:07, Usama Arif wrote:
> > From: Brian Gerst <brgerst@...il.com>
> >
> > Eliminating global variables from the CPU startup path in order to simplify
> > it and facilitate parallel startup.
>
> As this patch is now part of the parallel boot series and actually
> introduces smpboot_control, the above is neither accurate nor useful.
>
> Folks, really.
>
> > Remove initial_stack, and load RSP from current_task->thread.sp instead.
>
>
> >  #ifdef CONFIG_SMP
> > -     initial_stack = (unsigned long)temp_stack + sizeof(temp_stack);
> > +     current->thread.sp = (unsigned long)temp_stack + sizeof(temp_stack);
>
> This lacks a comment about the temporary (ab)use of current->thread.sp

task->thread.sp represents the saved stack pointer of a sleeping or
newly forked task (see __switch_to_asm()), and the boot cpu's idle
task is effectively going to sleep.  Using a temporary stack for
resume is kind of a hack to workaround the fact that the idle task
stack is in use already, but improving that is out of scope for this
series.

--
Brian Gerst