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

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Message-ID: <df39d7d9-4291-450b-85b7-4c57a3c4a025@amd.com>
Date: Wed, 18 Jun 2025 10:56:11 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Naveen N Rao <naveen@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, linux-kernel@...r.kernel.org, "H. Peter Anvin"
 <hpa@...or.com>, Sairaj Kodilkar <sarunkod@....com>,
 Mario Limonciello <mario.limonciello@....com>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 "Gautham R. Shenoy" <gautham.shenoy@....com>, Babu Moger
 <babu.moger@....com>, Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Subject: Re: [PATCH 2/2] x86/cpu/topology: Use initial APICID from XTOPOEXT on
 AMD/HYGON

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@...nel.org>

Thank you for debugging the issue and testing :)

-- 
Thanks and Regards,
Prateek