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. ] 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> Tested-by: Michal Pecio <michal.pecio@gmail.com> Tested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com> Cc: stable@vger.kernel.org Link: https://lore.kernel.org/20251111145357.4031846-1-yazen.ghannam@amd.com