The following commit has been merged into the x86/boot branch of tip:

Commit-ID:     825c5d67c389426b71710d89dd9cb428e6a7e81e
Gitweb:        https://git.kernel.org/tip/825c5d67c389426b71710d89dd9cb428e6a7e81e
Author:        Yazen Ghannam <yazen.ghannam@amd.com>
AuthorDate:    Tue, 11 Nov 2025 14:53:57 
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Sat, 13 Dec 2025 11:00:28 +01:00

x86/acpi/boot: Correct acpi_is_processor_usable() check again

ACPI v6.3 defined a new "Online Capable" MADT LAPIC flag. This bit is
used in conjunction with the "Enabled" MADT LAPIC flag to determine if
a CPU can be enabled/hotplugged by the OS after boot.

Before the new bit was defined, the "Enabled" bit was explicitly
described like this (ACPI v6.0 wording provided):

  "If zero, this processor is unusable, and the operating system
  support will not attempt to use it"

This means that CPU hotplug (based on MADT) is not possible. Many BIOS
implementations follow this guidance. They may include LAPIC entries in
MADT for unavailable CPUs, but since these entries are marked with
"Enabled=0" it is expected that the OS will completely ignore these
entries.

However, QEMU will do the same (include entries with "Enabled=0") for
the purpose of allowing CPU hotplug within the guest.

Comment from QEMU function pc_madt_cpu_entry():

  /* ACPI spec says that LAPIC entry for non present
   * CPU may be omitted from MADT or it must be marked
   * as disabled. However omitting non present CPU from
   * MADT breaks hotplug on linux. So possible CPUs
   * should be put in MADT but kept disabled.
   */

Recent Linux topology changes broke the QEMU use case. A following fix
for the QEMU use case broke bare metal topology enumeration.

Rework the Linux MADT LAPIC flags check to allow the QEMU use case only
for guests and to maintain the ACPI spec behavior for bare metal.

Remove an unnecessary check added to fix a bare metal case introduced by
the QEMU "fix".

  [ bp: Change logic as Michal suggested. ]
  [ mingo: Removed misapplied -stable tag. ]

Fixes: fed8d8773b8e ("x86/acpi/boot: Correct acpi_is_processor_usable() check")
Fixes: f0551af02130 ("x86/topology: Ignore non-present APIC IDs in a present package")
Closes: https://lore.kernel.org/r/20251024204658.3da9bf3f.michal.pecio@gmail.com
Reported-by: Michal Pecio <michal.pecio@gmail.com>
Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Michal Pecio <michal.pecio@gmail.com>
Tested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Link: https://lore.kernel.org/20251111145357.4031846-1-yazen.ghannam@amd.com
---
 arch/x86/kernel/acpi/boot.c    | 12 ++++++++----
 arch/x86/kernel/cpu/topology.c | 15 ---------------
 2 files changed, 8 insertions(+), 19 deletions(-)

diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
index 9fa321a..d6138b2 100644
--- a/arch/x86/kernel/acpi/boot.c
+++ b/arch/x86/kernel/acpi/boot.c
@@ -35,6 +35,7 @@
 #include <asm/smp.h>
 #include <asm/i8259.h>
 #include <asm/setup.h>
+#include <asm/hypervisor.h>
 
 #include "sleep.h" /* To include x86_acpi_suspend_lowlevel */
 static int __initdata acpi_force = 0;
@@ -164,11 +165,14 @@ static bool __init acpi_is_processor_usable(u32 lapic_flags)
 	if (lapic_flags & ACPI_MADT_ENABLED)
 		return true;
 
-	if (!acpi_support_online_capable ||
-	    (lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
-		return true;
+	if (acpi_support_online_capable)
+		return lapic_flags & ACPI_MADT_ONLINE_CAPABLE;
 
-	return false;
+	/*
+	 * QEMU expects legacy "Enabled=0" LAPIC entries to be counted as usable
+	 * in order to support CPU hotplug in guests.
+	 */
+	return !hypervisor_is_type(X86_HYPER_NATIVE);
 }
 
 static int __init
diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
index f55ea3c..23190a7 100644
--- a/arch/x86/kernel/cpu/topology.c
+++ b/arch/x86/kernel/cpu/topology.c
@@ -27,7 +27,6 @@
 #include <xen/xen.h>
 
 #include <asm/apic.h>
-#include <asm/hypervisor.h>
 #include <asm/io_apic.h>
 #include <asm/mpspec.h>
 #include <asm/msr.h>
@@ -236,20 +235,6 @@ 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++;
-			return;
-		}
-
 		topo_info.nr_disabled_cpus++;
 	}