linux-kernel - Re: [PATCH v3 06/12] x86/amd_nb: Use topology info to get AMD node count

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Message-ID: <20251023183154.1e807af6.michal.pecio@gmail.com>
Date: Thu, 23 Oct 2025 18:31:54 +0200
From: Michal Pecio <michal.pecio@...il.com>
To: Yazen Ghannam <yazen.ghannam@....com>
Cc: Shyam-sundar.S-k@....com, bhelgaas@...gle.com, hdegoede@...hat.com,
 ilpo.jarvinen@...ux.intel.com, jdelvare@...e.com,
 linux-edac@...r.kernel.org, linux-hwmon@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
 linux@...ck-us.net, mario.limonciello@....com,
 naveenkrishna.chatradhi@....com, platform-driver-x86@...r.kernel.org,
 suma.hegde@....com, tony.luck@...el.com, x86@...nel.org
Subject: Re: [PATCH v3 06/12] x86/amd_nb: Use topology info to get AMD node
 count

On Thu, 23 Oct 2025 12:09:06 -0400, Yazen Ghannam wrote:
> On Thu, Oct 23, 2025 at 05:01:07PM +0200, Michal Pecio wrote:
> > On Thu, 23 Oct 2025 09:59:35 -0400, Yazen Ghannam wrote:  
> > > Thanks Michal.
> > > 
> > > I don't see anything obviously wrong.  
> > 
> > Which code is responsible for setting up those bitmaps which
> > are counted by topology_init_possible_cpus()?
> > 
> > I guess I could add some printks there and reboot.
> >   
> 
> The kernel seems to think there are 6 CPUs on your system:
> 
> [    0.072059] CPU topo: Allowing 4 present CPUs plus 2 hotplug CPUs

I thought this is because I have NR_CPUS set to 6, as this config
originally came from the X6 machine, but I am not sure.

> 
> We don't seem them enabled, but they may still get APIC IDs. If so, then
> the IDs would be beyond the core shift of 2.
> 
> APIC IDs b'0 00 -> CPU0 on logical package 0
> 	 b'0 01 -> CPU1 on logical package 0
> 	 b'0 10 -> CPU2 on logical package 0
> 	 b'0 11 -> CPU3 on logical package 0
> 	 b'1 00 -> CPU0 on logical package 1
> 	 b'1 01 -> CPU1 on logical package 1
> 
> 
> Please try booting with "possible_cpus=4".

OK, will try it next time I'm rebooting.

> The "number of possible CPUs" comes from the ACPI Multiple APIC
> Description Table (MADT). This has the signature "APIC".
> 
> Can you please provide the disassembly of this table?

Interesting, it looks like there are indeed 6 LAPICs there.
BIOS bug? Attaching apic.dsl.

> Can you please share the dmesg output from that system? And the ACPI
> table too?

Will try later but I don't recall any anomalies there.
I remember checking the topology output and it made sense:
1 package, 1 die, 6 cores, 6 threads.

View attachment "apic.dsl" of type "text/x-dsl" (5144 bytes)