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

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Message-ID: <20250612072921.15107-3-kprateek.nayak@amd.com>
Date: Thu, 12 Jun 2025 07:29:21 +0000
From: K Prateek Nayak <kprateek.nayak@....com>
To: 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>
CC: "H. Peter Anvin" <hpa@...or.com>, Naveen rao <naveen.rao@....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>, K Prateek Nayak
	<kprateek.nayak@....com>
Subject: [PATCH 2/2] x86/cpu/topology: Use initial APICID from XTOPOEXT on AMD/HYGON

Prior to the topology parsing rewrite and the switchover to the new
parsing logic for AMD processors in commit c749ce393b8f ("x86/cpu: Use
common topology code for AMD"), the "initial_apicid" on these platforms
was:

- First initialized to the LocalApicId from CPUID leaf 0x1 EBX[31:24].

- Then overwritten by the ExtendedLocalApicId in CPUID leaf 0xb
  EDX[31:0] on processors that supported topoext.

With the new parsing flow introduced in commit f7fb3b2dd92c ("x86/cpu:
Provide an AMD/HYGON specific topology parser"), parse_8000_001e() now
unconditionally overwrites the "initial_apicid" already parsed during
cpu_parse_topology_ext().

Although this has not been a problem on baremetal platforms, on
virtualized AMD guests that feature more than 255 cores, QEMU 0's out
the CPUID leaf 0x8000001e on CPUs with "CoreID" > 255 to prevent
collision of these IDs in EBX[7:0] which can only represent a maximum of
255 cores [1].

This results in the following FW_BUG being logged when booting a guest
with more than 255 cores:

    [Firmware Bug]: CPU 512: APIC ID mismatch. CPUID: 0x0000 APIC: 0x0200

Rely on the APICID parsed during cpu_parse_topology_ext() from CPUID
leaf 0x80000026 or 0xb and only use the APICID from leaf 0x8000001e if
cpu_parse_topology_ext() failed (has_topoext is false).

On platforms that support the 0xb leaf (Zen2 or later, AMD guests on
QEMU) or the extended leaf 0x80000026 (Zen4 or later), the
"initial_apicid" is now set to the value parsed from EDX[31:0].

On older AMD/Hygon platforms that does not support the 0xb leaf but
supports the TOPOEXT extension (Fam 0x15, 0x16, 0x17[Zen1], and Hygon),
the current behavior is retained where "initial_apicid" is set using
the 0x8000001e leaf.

Link: https://github.com/qemu/qemu/commit/35ac5dfbcaa4b [1]
Debugged-by: Naveen N Rao (AMD) <naveen@...nel.org>
Debugged-by: Sairaj Kodilkar <sarunkod@....com>
Fixes: c749ce393b8f ("x86/cpu: Use common topology code for AMD")
Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
---
 arch/x86/kernel/cpu/topology_amd.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

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;

 	/*
 	 * If leaf 0xb is available, then the domain shifts are set
-- 
2.34.1