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

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Date:   Wed, 01 Mar 2023 11:18:24 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Woodhouse <dwmw2@...radead.org>,
        Usama Arif <usama.arif@...edance.com>, kim.phillips@....com,
        brgerst@...il.com
Cc:     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
Subject: Re: [PATCH v12 06/11] x86/smpboot: Remove initial_stack on 64-bit

On Tue, Feb 28 2023 at 20:43, David Woodhouse wrote:
> On 28 February 2023 20:17:19 GMT, Thomas Gleixner <tglx@...utronix.de> wrote:
>>> +	 * Make the resuming CPU use the temporary stack at startup
>>> +	 * by setting current->thread.sp to point to that. The true
>>> +	 * %rsp will be restored with the rest of the CPU context,
>>> +	 * by do_suspend_lowlevel().
>>
>>Right, but what restores current->thread.sp? thread.sp is used by
>>unwinders...
>
> Unwinding a thread that is actually *on* the CPU?

No.

> By the time it's taken off, won't ->thread.sp have been written out
> again? I figured it was just a dead variable while the actual %rsp was
> in use?

Yes. It's not used when the thread is on the CPU. And you are right,
it's saved and restored in switch_to(). Can you please add a comment to
that effect?

Thanks,

        tglx