linux-kernel - Re: [patch 37/53] x86/cpu: Detect real BSP on crash kernels

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Date: Mon, 15 Jan 2024 10:41:49 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: "Zhang, Rui" <rui.zhang@...el.com>, "Brown, Len" <len.brown@...el.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc: "jgross@...e.com" <jgross@...e.com>, "x86@...nel.org" <x86@...nel.org>,
 "arjan@...ux.intel.com" <arjan@...ux.intel.com>, "kprateek.nayak@....com"
 <kprateek.nayak@....com>, "Tang, Feng" <feng.tang@...el.com>,
 "kan.liang@...ux.intel.com" <kan.liang@...ux.intel.com>,
 "thomas.lendacky@....com" <thomas.lendacky@....com>, "ray.huang@....com"
 <ray.huang@....com>, "andrew.cooper3@...rix.com"
 <andrew.cooper3@...rix.com>, "Sivanich, Dimitri"
 <dimitri.sivanich@....com>, "paulmck@...nel.org" <paulmck@...nel.org>,
 "Mehta, Sohil" <sohil.mehta@...el.com>, "andy@...radead.org"
 <andy@...radead.org>
Subject: Re: [patch 37/53] x86/cpu: Detect real BSP on crash kernels

On Sat, Jan 13 2024 at 07:35, Zhang, Rui wrote:
> On Fri, 2024-01-12 at 16:39 +0100, Thomas Gleixner wrote:
>> It brings the effect that we can detect when we are not booting
>> (kexec
>> case) on the actual boot CPU because then the first enumerated APIC
>> ID
>> is not the same as the boot CPU APIC ID. No?
>
> Right.
> I was thinking in the way this patch series does, which just compares
> the boot CPU APIC ID and the lowest numbered APIC ID.

Yes, that's obviously not working when the lowest APIC ID is not the
actual boot CPU.

>> I agree that MADT is error prone too given the fact that not even
>> Intel
>> can get it right....
>
> For this MTL, I can raise an internal ticket to get it right.
>
> Are there quite some platforms with BSP not listed as the first entry
> in MADT?
> if so, we still have to live with the kexec boot parameter? :)

I haven't found one yet, but you might look through the P/E systems to
get a picture.