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

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Date: Tue, 9 Jan 2024 01:54:17 +0000
From: "Zhang, Rui" <rui.zhang@...el.com>
To: "tglx@...utronix.de" <tglx@...utronix.de>, "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 Mon, 2024-01-08 at 17:13 +0100, Thomas Gleixner wrote:
> On Mon, Jan 08 2024 at 15:54, Thomas Gleixner wrote:
> 
> > On Mon, Jan 08 2024 at 14:11, Zhang, Rui wrote:
> > > > +static __init void check_for_kdump_kernel(void)
> > > > +{
> > > > +       u32 bsp_apicid;
> > > > +
> > > > +       /*
> > > > +        * There is no real good way to detect whether this a
> > > > kdump()
> > > > +        * kernel, but except on the Voyager SMP monstrosity
> > > > which is
> > > > not
> > > > +        * longer supported, the real BSP has always the lowest
> > > > numbered
> > > > +        * APIC ID. If a crash happened on an AP, which then
> > > > ends up
> > > > as
> > > > +        * boot CPU in the kdump() kernel, then sending INIT to
> > > > the
> > > > real
> > > > +        * BSP would reset the whole system.
> > > > +        */
> > > 
> > > 
> > > Hi, Thomas,
> > > 
> > > Unfortunately this causes a regression on Intel Meteorlake
> > > platform,
> > > where the BSP APIC ID is NOT the lowest numbered APIC ID
> > > (instead,
> > > CPU12, the first Ecore CPU, has APIC ID 0).
> > 
> > Bah. Let me think about that.
> 
> In which order are the APICs/CPUs enumerated by ACPI?


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
[050h 0080   4]          Processor x2Apic ID : 00000018
[060h 0096   4]          Processor x2Apic ID : 00000019
[070h 0112   4]          Processor x2Apic ID : 00000020
[080h 0128   4]          Processor x2Apic ID : 00000021
[090h 0144   4]          Processor x2Apic ID : 00000028
[0A0h 0160   4]          Processor x2Apic ID : 00000029
[0B0h 0176   4]          Processor x2Apic ID : 00000030
[0C0h 0192   4]          Processor x2Apic ID : 00000031
[0D0h 0208   4]          Processor x2Apic ID : 00000038
[0E0h 0224   4]          Processor x2Apic ID : 00000039
[0F0h 0240   4]          Processor x2Apic ID : 00000000
[100h 0256   4]          Processor x2Apic ID : 00000002
[110h 0272   4]          Processor x2Apic ID : 00000004
[120h 0288   4]          Processor x2Apic ID : 00000006
[130h 0304   4]          Processor x2Apic ID : 00000008
[140h 0320   4]          Processor x2Apic ID : 0000000A
[150h 0336   4]          Processor x2Apic ID : 0000000C
[160h 0352   4]          Processor x2Apic ID : 0000000E
[170h 0368   4]          Processor x2Apic ID : 00000040
[180h 0384   4]          Processor x2Apic ID : 00000042

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)
      x2APIC ID of logical processor = 0x11 (17)
      x2APIC ID of logical processor = 0x18 (24)
      x2APIC ID of logical processor = 0x19 (25)
      x2APIC ID of logical processor = 0x21 (33)
      x2APIC ID of logical processor = 0x28 (40)
      x2APIC ID of logical processor = 0x29 (41)
      x2APIC ID of logical processor = 0x30 (48)
      x2APIC ID of logical processor = 0x31 (49)
      x2APIC ID of logical processor = 0x38 (56)
      x2APIC ID of logical processor = 0x39 (57)
      x2APIC ID of logical processor = 0x0 (0)
      x2APIC ID of logical processor = 0x2 (2)
      x2APIC ID of logical processor = 0x4 (4)
      x2APIC ID of logical processor = 0x6 (6)
      x2APIC ID of logical processor = 0x8 (8)
      x2APIC ID of logical processor = 0xa (10)
      x2APIC ID of logical processor = 0xc (12)
      x2APIC ID of logical processor = 0xe (14)
      x2APIC ID of logical processor = 0x40 (64)
      x2APIC ID of logical processor = 0x42 (66)

thanks,
rui