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 <20230226110802.103134-7-usama.arif@bytedance.com> <87k001n4xo.ffs@tglx>
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From:   Brian Gerst <brgerst@gmail.com>
Date:   Tue, 28 Feb 2023 17:22:46 -0500
Message-ID: <CAMzpN2iBjzKxrqG97ck8wUh-jViDJCOUcqE02gA3g_BiDBA2Ug@mail.gmail.com>
Subject: Re: [PATCH v12 06/11] x86/smpboot: Remove initial_stack on 64-bit
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Usama Arif <usama.arif@bytedance.com>, dwmw2@infradead.org,
        kim.phillips@amd.com, piotrgorski@cachyos.org,
        oleksandr@natalenko.name, arjan@linux.intel.com, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com,
        x86@kernel.org, pbonzini@redhat.com, paulmck@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        rcu@vger.kernel.org, mimoja@mimoja.de, hewenliang4@huawei.com,
        thomas.lendacky@amd.com, seanjc@google.com, pmenzel@molgen.mpg.de,
        fam.zheng@bytedance.com, punit.agrawal@bytedance.com,
        simon.evans@bytedance.com, liangma@liangbit.com,
        David Woodhouse <dwmw@amazon.co.uk>
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On Tue, Feb 28, 2023 at 11:13=E2=80=AFAM Thomas Gleixner <tglx@linutronix.d=
e> wrote:
>
> On Sun, Feb 26 2023 at 11:07, Usama Arif wrote:
> > From: Brian Gerst <brgerst@gmail.com>
> >
> > Eliminating global variables from the CPU startup path in order to simp=
lify
> > 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 =3D (unsigned long)temp_stack + sizeof(temp_stack);
> > +     current->thread.sp =3D (unsigned long)temp_stack + sizeof(temp_st=
ack);
>
> 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