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

Zhang, Rui
2024-01-09T01:54:27Z

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