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: <20250820085935.GBaKWOd5Wk3plH0h1l@fat_crate.local>
Date: Wed, 20 Aug 2025 10:59:35 +0200
From: Borislav Petkov <bp@...en8.de>
To: Naveen N Rao <naveen@...nel.org>
Cc: K Prateek Nayak <kprateek.nayak@....com>,
	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

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...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette