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: <20250819212436.GIaKTrlN6tjmuXJvxs@fat_crate.local>
Date: Tue, 19 Aug 2025 23:24:36 +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>,
	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 Tue, Aug 19, 2025 at 07:58:52PM +0530, K Prateek Nayak wrote:
> This is possible, however what should be the right thing for
> CPUID_Fn8000001E_EBX [Core Identifiers] (Core::X86::Cpuid::CoreId)?
> 
> Should QEMU just wrap and start counting the Core Identifiers again
> from 0?
> 
> Or Should QEMU go ahead and populate just the
> CPUID_Fn8000001E_EAX [Extended APIC ID] (Core::X86::Cpuid::ExtApicId)
> fields and continue to zero out EBX and ECX when CoreID > 255?

I think the right thing to do is what the HW does (or will do), when it gets
to more than 256 APIC IDs - "cores" is ambiguous.

Perhaps something to discuss with hw folks internally first and then stick to
that plan everywhere, qemu included.

-- 
Regards/Gruss,
    Boris.

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