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Yazen Ghannam a675401270 x86/acpi/boot: Correct acpi_is_processor_usable() check again 
[ Upstream commit adbf61cc47 ]

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: fed8d8773b ("x86/acpi/boot: Correct acpi_is_processor_usable() check")
Fixes: f0551af021 ("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
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2026-03-12 07:09:23 -04:00
arch x86/acpi/boot: Correct acpi_is_processor_usable() check again 2026-03-12 07:09:23 -04:00
block block: decouple secure erase size limit from discard size limit 2026-03-04 07:19:38 -05:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: authencesn - reject too-short AAD (assoclen<8) to match ESP/ESN spec 2026-01-30 10:32:12 +01:00
Documentation ipv6: Move ipv6_fl_list from ipv6_pinfo to inet_sock. 2026-03-04 07:20:44 -05:00
drivers spi: stm32: fix missing pointer assignment in case of dma chaining 2026-03-12 07:09:22 -04:00
fs btrfs: fix compat mask in error messages in btrfs_check_features() 2026-03-12 07:09:20 -04:00
include bpf: Introduce tnum_step to step through tnum's members 2026-03-12 07:09:23 -04:00
init printk changes for 6.18 2025-10-04 11:13:11 -07:00
io_uring io_uring/zcrx: fix user_ref race between scrub and refill paths 2026-03-04 07:21:33 -05:00
ipc ipc: don't audit capability check in ipc_permissions() 2026-02-26 14:59:19 -08:00
kernel bpf: Improve bounds when tnum has a single possible value 2026-03-12 07:09:23 -04:00
lib debugobject: Make it work with deferred page initialization - again 2026-03-12 07:09:08 -04:00
LICENSES LICENSES: Replace the obsolete address of the FSF in the GFDL-1.2 2025-07-24 11:15:39 +02:00
mm mm: numa_memblks: Identify the accurate NUMA ID of CFMW 2026-03-04 07:21:31 -05:00
net net: nfc: nci: Fix parameter validation for packet data 2026-03-04 07:21:34 -05:00
rust rust: list: Add unsafe blocks for container_of and safety comments 2026-03-04 07:21:35 -05:00
samples samples/ftrace: Adjust LoongArch register restore order in direct calls 2026-01-08 10:17:11 +01:00
scripts kbuild: rpm-pkg: Disable automatic requires for manual debuginfo package 2026-03-04 07:21:32 -05:00
security ima: verify the previous kernel's IMA buffer lies in addressable RAM 2026-03-04 07:21:17 -05:00
sound ASoC: SDCA: Fix comments for sdca_irq_request() 2026-03-12 07:09:21 -04:00
tools selftests/bpf: Fix OOB read in dmabuf_collector 2026-03-12 07:09:21 -04:00
usr gen_init_cpio: Ignore fsync() returning EINVAL on pipes 2025-10-07 09:53:05 -07:00
virt KVM: Don't clobber irqfd routing type when deassigning irqfd 2026-02-11 13:41:44 +01:00
.clang-format memblock: drop for_each_free_mem_pfn_range_in_zone_from() 2025-09-14 08:49:03 +03:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: remove Alyssa Rosenzweig 2025-09-18 21:17:31 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore .gitignore: ignore compile_commands.json globally 2025-08-12 15:53:55 -07:00
.mailmap 8 hotfixes. 4 are cc:stable, 7 are against mm/. 2025-11-26 12:38:05 -08:00
.pylintrc tools: docs: parse-headers.py: move it from sphinx dir 2025-08-29 15:54:42 -06:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: mark ISDN subsystem as orphan 2025-10-27 17:49:45 -07:00
Kbuild sched: Make migrate_{en,dis}able() inline 2025-09-25 09:57:16 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS soc: fixes for 6.18, part 4 2025-11-28 09:57:31 -08:00
Makefile Linux 6.18.16 2026-03-04 07:25:12 -05:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.