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

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Message-ID: <5764e711-4c3f-4476-9ecb-1f7643e3b60d@amd.com>
Date: Thu, 23 Oct 2025 11:22:29 -0500
From: Mario Limonciello <mario.limonciello@....com>
To: Yazen Ghannam <yazen.ghannam@....com>,
 Michal Pecio <michal.pecio@...il.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,
 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 10/23/25 11:09 AM, 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
> 
> 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".
> 
> 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?
> 
> You can use the following commands:
> 1) Dump the ACPI tables to binaries:	"sudo acpidump -b"
> 2) Disassemble the APIC table:		"iasl -d apic.dat"
> 
> Both commands are part of the "acpica-tools" package.
> 
> [...]
>>
>> BTW, I forgot to mention that I have a second seemingly identical
>> board with same BIOS running Phenom X6 1090T. It is not affected.
>> Not sure if this is helpful. I haven't tried swapping CPUs.
> 
> Can you please share the dmesg output from that system? And the ACPI
> table too?
> 
> If the BIOS is the same, then I wonder if they hardcoded 6 CPUs in the
> MADT then mark the extras as "not enabled" on parts with lower cores.
> 
> Thanks,
> Yazen

As this is an ancient BIOS this reminds me of some related commits:

aa06e20f1be6 ("x86/ACPI: Don't add CPUs that are not online capable")
a74fabfbd1b70 ("x86/ACPI/boot: Use FADT version to check support for 
online capable")

Does reverting that second one help?