From:   Thomas Gleixner <tglx@linutronix.de>
To:     David Woodhouse <dwmw2@infradead.org>,
        Usama Arif <usama.arif@bytedance.com>, kim.phillips@amd.com,
        brgerst@gmail.com
Cc:     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
Subject: Re: [PATCH v12 06/11] x86/smpboot: Remove initial_stack on 64-bit
In-Reply-To: <64B60F62-3760-43A0-A0FB-C349DA70C013@infradead.org>
References: <20230226110802.103134-1-usama.arif@bytedance.com>
 <20230226110802.103134-7-usama.arif@bytedance.com> <87k001n4xo.ffs@tglx>
 <c6863590f5fbf139f6aec50d0f3bc8e8b00cfcaf.camel@infradead.org>
 <87edq9mto0.ffs@tglx> <64B60F62-3760-43A0-A0FB-C349DA70C013@infradead.org>
Date:   Wed, 01 Mar 2023 11:18:24 +0100
Message-ID: <87y1oglqq7.ffs@tglx>
MIME-Version: 1.0
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On Tue, Feb 28 2023 at 20:43, David Woodhouse wrote:
> On 28 February 2023 20:17:19 GMT, Thomas Gleixner <tglx@linutronix.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