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Message-ID: <20251024213204.GA311478@yaz-khff2.amd.com>
Date: Fri, 24 Oct 2025 17:32:04 -0400
From: Yazen Ghannam <yazen.ghannam@....com>
To: Michal Pecio <michal.pecio@...il.com>
Cc: x86@...nel.org, regressions@...ts.linux.dev,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Mario Limonciello <mario.limonciello@....com>,
Eric DeVolder <eric.devolder@...cle.com>,
linux-kernel@...r.kernel.org
Subject: Re: AMD topology broken on various 754/AM2+/AM3/AM3+ systems causes
NB/EDAC/GART regression since 6.14
On Fri, Oct 24, 2025 at 08:46:58PM +0200, Michal Pecio wrote:
> Hi,
>
> This report is related to discussion here:
> https://lore.kernel.org/all/20251022011610.60d0ba6e.michal.pecio@gmail.com/
>
> Commit bc7b2e629e0c ("x86/amd_nb: Use topology info to get AMD node
> count") bails out if it can't find the NB of each node reportedy by
> topology. Then NB features like EDAC or GART IOMMU aren't available.
>
> Which was maybe not a bad idea, nobody expects those things to work
> on selected nodes only. (I think?) But it relies on the optimistic
> assumption that topology knows the true number of nodes.
>
> Today I tested 5 older AMD64 systems with socket 754/AM2+/AM3/AM3+
> on MSI/ASUS motherboards. *All* of them report more than one node if
> the CPU has fewer cores than supported by the BIOS.
>
> (I also have one AM4 system which is OK, but can't speak for others).
>
> This is due to peculiarity of their MADT tables - they report as many
> LAPICs as the BIOS can support and excess LAPICs are simply disabled.
> FWIW, it's also a pattern that disabled APIC IDs have 0x80 bit set.
>
> The kernel counts this as "hotpluggable CPUs", since supposedly it's
> indistinguishable from actual multi-socket systems before ACPI 6.3,
> where the "online capable" flag was added to disambiguate hotplug and
> nonexistent but theoretically supported CPUs.
>
> Or at least that's what commit fed8d8773b8e ("x86/acpi/boot: Correct
> acpi_is_processor_usable() check") seems to imply.
>
> On pre-ACPI 6.3 systems those disabled LAPICs inflate topology size
> and result in breakage on recent kernels. A few examples below give
> an idea what those MADTs look like and how the kernel reads them.
>
> Regards,
> Michal
>
>
> Athlon 3000+ on S754:
>
> [02Fh 0047 001h] Local Apic ID : 00
> [030h 0048 004h] Flags (decoded below) : 00000001 # enabled
> --
> [037h 0055 001h] Local Apic ID : 81
> [038h 0056 004h] Flags (decoded below) : 00000000
>
> [ 0.027690] CPU topo: Max. logical packages: 2
> [ 0.027691] CPU topo: Max. logical dies: 2
> [ 0.027692] CPU topo: Max. dies per package: 1
> [ 0.027703] CPU topo: Max. threads per core: 1
> [ 0.027704] CPU topo: Num. cores per package: 1
> [ 0.027705] CPU topo: Num. threads per package: 1
> [ 0.027706] CPU topo: Allowing 1 present CPUs plus 1 hotplug CPUs
>
> Athlon II X2 250 on AM3+:
>
> [02Fh 0047 001h] Local Apic ID : 00
> [030h 0048 004h] Flags (decoded below) : 00000001 # enabled
> --
> [037h 0055 001h] Local Apic ID : 01
> [038h 0056 004h] Flags (decoded below) : 00000001 # enabled
> --
> [03Fh 0063 001h] Local Apic ID : 82
> [040h 0064 004h] Flags (decoded below) : 00000000
> --
> [047h 0071 001h] Local Apic ID : 83
> [048h 0072 004h] Flags (decoded below) : 00000000
> --
> [04Fh 0079 001h] Local Apic ID : 84
> [050h 0080 004h] Flags (decoded below) : 00000000
> --
> [057h 0087 001h] Local Apic ID : 85
> [058h 0088 004h] Flags (decoded below) : 00000000
> --
> [05Fh 0095 001h] Local Apic ID : 86
> [060h 0096 004h] Flags (decoded below) : 00000000
> --
> [067h 0103 001h] Local Apic ID : 87
> [068h 0104 004h] Flags (decoded below) : 00000000
>
> [ 0.147372] CPU topo: Max. logical packages: 3 # not sure why not 4
> [ 0.147372] CPU topo: Max. logical dies: 3
> [ 0.147373] CPU topo: Max. dies per package: 1
> [ 0.147379] CPU topo: Max. threads per core: 1
> [ 0.147379] CPU topo: Num. cores per package: 2
> [ 0.147380] CPU topo: Num. threads per package: 2
> [ 0.147381] CPU topo: Allowing 2 present CPUs plus 6 hotplug CPUs
>
> Phenom II X4 965 on AM3:
>
> [02Fh 0047 1] Local Apic ID : 00
> [030h 0048 4] Flags (decoded below) : 00000001 # enabled
> --
> [037h 0055 1] Local Apic ID : 01
> [038h 0056 4] Flags (decoded below) : 00000001 # enabled
> --
> [03Fh 0063 1] Local Apic ID : 02
> [040h 0064 4] Flags (decoded below) : 00000001 # enabled
> --
> [047h 0071 1] Local Apic ID : 03
> [048h 0072 4] Flags (decoded below) : 00000001 # enabled
> --
> [04Fh 0079 1] Local Apic ID : 84
> [050h 0080 4] Flags (decoded below) : 00000000
> --
> [057h 0087 1] Local Apic ID : 85
> [058h 0088 4] Flags (decoded below) : 00000000
>
> [ 0.072112] CPU topo: Max. logical packages: 2
> [ 0.072112] CPU topo: Max. logical dies: 2
> [ 0.072113] CPU topo: Max. dies per package: 1
> [ 0.072118] CPU topo: Max. threads per core: 1
> [ 0.072118] CPU topo: Num. cores per package: 4
> [ 0.072119] CPU topo: Num. threads per package: 4
> [ 0.072120] CPU topo: Allowing 4 present CPUs plus 2 hotplug CPUs
So far, I think the way to go is add explicit quirk for known issues.
Please see the patch below.
Thanks,
Yazen
>From eeb0745e973055d8840b536cfa842d6f2bf4ac52 Mon Sep 17 00:00:00 2001
From: Yazen Ghannam <yazen.ghannam@....com>
Date: Fri, 24 Oct 2025 21:19:26 +0000
Subject: [PATCH] x86/topology: Add helper to ignore bogus MADT entries
Some older Intel and AMD systems include bogus ACPI MADT entries. These
entries show as "disabled". And it's not clear if they are physically
present but offline, i.e halted. Or if they are not physically present
at all.
Ideally, if they are not physically present, then they should not be
listed in MADT. There doesn't seem to be any explicit x86 topology info
that can be used to verify if the entries are bogus or not.
Add a helper function to collect vendor-specific checks to ignore bogus
APIC IDs. Start with known quirks for an Intel SNB model and older AMD
K10 models.
Fixes: f0551af02130 ("x86/topology: Ignore non-present APIC IDs in a present package")
Signed-off-by: Yazen Ghannam <yazen.ghannam@....com>
---
arch/x86/kernel/cpu/topology.c | 52 ++++++++++++++++++++++++++--------
1 file changed, 40 insertions(+), 12 deletions(-)
diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
index 6073a16628f9..704788b92395 100644
--- a/arch/x86/kernel/cpu/topology.c
+++ b/arch/x86/kernel/cpu/topology.c
@@ -219,6 +219,45 @@ static unsigned int topo_unit_count(u32 lvlid, enum x86_topology_domains at_leve
return cnt;
}
+/*
+ * Some older BIOSes include extra entries in MADT.
+ * Do some vendor-specific checks to ignore them.
+ */
+static bool ignore_extra_apic_entry(u32 apic_id)
+{
+ u32 pkgid = topo_apicid(apic_id, TOPO_PKG_DOMAIN);
+ struct cpuinfo_x86 *c = &boot_cpu_data;
+
+ /* Allow "physically not possible" cases if in a guest. */
+ if (!hypervisor_is_type(X86_HYPER_NATIVE))
+ return false;
+
+ /* This model only supports 8 threads in a package. */
+ if (c->x86_vendor == X86_VENDOR_INTEL &&
+ c->x86 == 0x6 && c->x86_model == 0x2d) {
+ if (topo_unit_count(pkgid, TOPO_PKG_DOMAIN, phys_cpu_present_map) >= 8)
+ goto reject;
+ }
+
+ /*
+ * Various older models have extra entries. A common trait is that the
+ * package ID derived from the APIC ID would be more than was ever supported.
+ */
+ if (c->x86_vendor == X86_VENDOR_AMD && c->x86 < 0x17) {
+ pkgid >>= x86_topo_system.dom_shifts[TOPO_PKG_DOMAIN - 1];
+
+ if (pkgid >= 8)
+ goto reject;
+ }
+
+ return false;
+
+reject:
+ pr_info_once("Ignoring hot-pluggable APIC ID %x.\n", apic_id);
+ topo_info.nr_rejected_cpus++;
+ return true;
+}
+
static __init void topo_register_apic(u32 apic_id, u32 acpi_id, bool present)
{
int cpu, dom;
@@ -240,19 +279,8 @@ static __init void topo_register_apic(u32 apic_id, u32 acpi_id, bool present)
cpuid_to_apicid[cpu] = apic_id;
topo_set_cpuids(cpu, apic_id, acpi_id);
} else {
- u32 pkgid = topo_apicid(apic_id, TOPO_PKG_DOMAIN);
-
- /*
- * Check for present APICs in the same package when running
- * on bare metal. Allow the bogosity in a guest.
- */
- if (hypervisor_is_type(X86_HYPER_NATIVE) &&
- topo_unit_count(pkgid, TOPO_PKG_DOMAIN, phys_cpu_present_map)) {
- pr_info_once("Ignoring hot-pluggable APIC ID %x in present package.\n",
- apic_id);
- topo_info.nr_rejected_cpus++;
+ if (ignore_extra_apic_entry(apic_id))
return;
- }
topo_info.nr_disabled_cpus++;
}
--
2.51.1
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