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

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Date: Wed, 10 Jan 2024 16:14:15 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: "Zhang, Rui" <rui.zhang@...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 Wed, Jan 10 2024 at 15:19, Thomas Gleixner wrote:
>> This is the order in MADT,
>> $ cat apic.dsl  | grep x2Apic
>> [030h 0048   4]          Processor x2Apic ID : 00000010
>> [040h 0064   4]          Processor x2Apic ID : 00000011
..
>> and this is the order in Linux (from CPU0 to CPUN)
>>       x2APIC ID of logical processor = 0x20 (32)
>>       x2APIC ID of logical processor = 0x10 (16)
>
> What a mess...

And clearly not according to the spec

  "The second is that platform firmware should list the boot processor
   as the first processor entry in the MADT."

Oh well. There are reasons why this is written the way it is.