Re: [PATCH 2/2] x86/cpu/topology: Use initial APICID from XTOPOEXT on AMD/HYGON

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Naveen,

On 6/18/2025 9:30 AM, Naveen N Rao wrote:
>> diff --git a/arch/x86/kernel/cpu/topology_amd.c b/arch/x86/kernel/cpu/topology_amd.c
>> index fec849fff82f..2ff6988e020a 100644
>> --- a/arch/x86/kernel/cpu/topology_amd.c
>> +++ b/arch/x86/kernel/cpu/topology_amd.c
>> @@ -80,7 +80,13 @@ static bool parse_8000_001e(struct topo_scan *tscan, bool has_topoext)
>>   
>>   	cpuid_leaf(0x8000001e, &leaf);
>>   
>> -	tscan->c->topo.initial_apicid = leaf.ext_apic_id;
>> +	/*
>> +	 * Prefer initial_apicid parsed from CPUID leaf 0x8000026 or 0xb
>> +	 * if available. Otherwise prefer the one from leaf 0x8000001e
>> +	 * over 0x1.
>> +	 */
>> +	if (!has_topoext)
>> +		tscan->c->topo.initial_apicid = leaf.ext_apic_id;
> 
> My understanding is that the rest of this function continues to work
> properly with an all-zero return value from CPUID leaf 0x8000001e - data
> from topoext is preferred where available, and other sources are used
> for llc_id.

Yes. QEMU just errs on the side of caution for CoreID collision in
0x8000001e and "NumSharingCache" in 0x8000001d EAX[25:14] can still
represent 2^12 (4096) CPUs in each cache domain which is larger than
the total number of vCPUs possible currently I believe.

> And, this fixes the firmware bug warning seen during a guest boot in
> qemu for me, so for this patch:
> Tested-by: Naveen N Rao (AMD) <naveen@kernel.org>

Thank you for debugging the issue and testing :)

-- 
Thanks and Regards,
Prateek