linux-kernel - Re: [PATCH v3 0/4] x86/cpu/topology: Work around the nuances of virtualization on AMD/Hygon

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Message-ID: <97aace4c-921f-4037-b8f2-c4112b4a26a9@amd.com>
Date: Wed, 20 Aug 2025 14:42:26 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Borislav Petkov <bp@...en8.de>, Naveen N Rao <naveen@...nel.org>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	"Dave Hansen" <dave.hansen@...ux.intel.com>, Sean Christopherson
	<seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, <x86@...nel.org>,
	Sairaj Kodilkar <sarunkod@....com>, "H. Peter Anvin" <hpa@...or.com>, "Peter
 Zijlstra (Intel)" <peterz@...radead.org>, "Xin Li (Intel)" <xin@...or.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>, Mario Limonciello
	<mario.limonciello@....com>, "Gautham R. Shenoy" <gautham.shenoy@....com>,
	Babu Moger <babu.moger@....com>, Suravee Suthikulpanit
	<suravee.suthikulpanit@....com>
Subject: Re: [PATCH v3 0/4] x86/cpu/topology: Work around the nuances of
 virtualization on AMD/Hygon

Hello Boris,

On 8/20/2025 2:29 PM, Borislav Petkov wrote:
> On Wed, Aug 20, 2025 at 01:41:28PM +0530, Naveen N Rao wrote:
>> That suggests use of leaf 0xb for the initial x2APIC ID especially 
>> during early init.  I'm not sure why leaf 0x8000001e was preferred over 
>> leaf 0xb in commit c749ce393b8f ("x86/cpu: Use common topology code for 
>> AMD") though.
> 
> Well, I see parse_topology_amd() calling cpu_parse_topology_ext() if you have
> TOPOEXT - which all AMD hw does - which then does cpu_parse_topology_ext() and
> that one tries 0x80000026 and then falls back to 0xb and *only* *then* to
> 0x8000001e.
> 
> So, it looks like it DTRT to me...

But parse_8000_001e() then unconditionally overwrites the
"initial_apicid" with the value in 0x8000001E EAX despite it being
populated from cpu_parse_topology_ext().

The flow is as follows:

  parse_topology_amd()
    if (X86_FEATURE_TOPOEXT) /* True */
      has_topoext = cpu_parse_topology_ext(); /* Populates "initial_apicid", returns True */

    /* parse_8000_0008() is never called since has_topoext is true */
    
    parse_8000_001e()
      if (!X86_FEATURE_TOPOEXT) /* False */
        return;

      /* Proceeds here */
      cpuid_leaf(0x8000001e, &leaf);
      tscan->c->topo.initial_apicid = leaf.ext_apic_id; /*** Overwritten here ***/

-- 
Thanks and Regards,
Prateek