linux-kernel - Re: [PATCH v4 0/4] x86/cpu/topology: Fix the preferred order of initial APIC ID parsing on AMD/Hygon

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Message-ID: <20250825084950.GAaKwjrvrmXZStqrji@fat_crate.local>
Date: Mon, 25 Aug 2025 10:49:50 +0200
From: Borislav Petkov <bp@...en8.de>
To: K Prateek Nayak <kprateek.nayak@....com>
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,
	Naveen rao <naveen.rao@....com>, 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>,
	Tom Lendacky <thomas.lendacky@....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>,
	Naveen N Rao <naveen@...nel.org>
Subject: Re: [PATCH v4 0/4] x86/cpu/topology: Fix the preferred order of
 initial APIC ID parsing on AMD/Hygon

On Mon, Aug 25, 2025 at 07:57:28AM +0000, K Prateek Nayak wrote:
> This led us down a rabbit hole of XTOPOLOGY vs TOPOEXT support, preferred

So in order to save people the rabbit hole wandering each time they (or we)
have to undertake, I think we should document what the whole logic and
precedences are wrt CPUID leafs and topology. What should be done where and so
on.

And those commit messages have a lot of text which explains that and I think
it would be worth the effort to start holding it down here
Documentation/arch/x86/topology.rst

No long texts, no big explanations - just the plain facts and what the current
strategy is wrt to which CPUID leafs we parse for what in what order and so
on.

You could start the AMD side, it doesn't have to be exhaustive - just the
facts from this rabbit hole trip.

And then we'll keep extending it and filling out the details so that it is
right there written down in one place.

Makes sense?

Thx.

-- 
Regards/Gruss,
    Boris.

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