[RFC PATCH v2 0/3] arm64: expose CPU prefetch and cache modulation controls

This series adds CONFIG_ARM64_CPUMOD, a default-off arm64 interface for
controlled performance characterization of selected implementation-defined
CPU prefetch and cache-management fields. It exposes a small set of named,
per-CPU attributes under the path below, with writable control fields
range-checked before register state is changed:

  /sys/devices/system/cpu/cpuN/cpumod/

The directory is created for online CPUs whose MIDR matches a recognized
Grace or Vera profile, with common and profile-specific attributes selected
for that CPU. Register accesses execute on the CPU that owns the sysfs
instance, and CPU hotplug creates or removes the per-CPU subtree as the CPU
transitions online or offline. Unsupported CPU profiles are skipped without
blocking module load or CPU hotplug.

The register-backed attributes are root-only read/write files. Reads return
the live value from the synchronous target-CPU callback rather than relying
on persistent per-attribute sysfs storage. affected_cpus remains
world-readable and read-only.

Changes since v1:
- Remove redundant GPL boilerplate from the new source file while retaining
  the SPDX identifier and NVIDIA copyright notice.
- Address Rob Herring's ABI feedback by adding standard
  Documentation/ABI/testing/sysfs-devices-system-cpu records and keeping the
  arm64 guide as a pointer to that ABI, without a second attribute list.
- Address Breno Leitao's access/readback feedback by returning live
  synchronous target-CPU register reads, changing the six register-backed
  controls to mode 0600, documenting affected_cpus as 0444, and adding
  deterministic selftest mode checks.

Koba Ko (3):
  arm64: add CPU prefetch and cache modulation sysfs interface
  Documentation/arch/arm64: document arm_cpumod
  selftests: arm64: add arm_cpumod kselftest

 .../ABI/testing/sysfs-devices-system-cpu      |  77 +++
 Documentation/arch/arm64/arm-cpumod.rst       |  66 +++
 Documentation/arch/arm64/index.rst            |   1 +
 arch/arm64/Kconfig                            |  17 +
 arch/arm64/kernel/Makefile                    |   1 +
 arch/arm64/kernel/arm_cpumod.c                | 458 +++++++++++++++
 arch/arm64/kernel/arm_cpumod_internal.h       |  93 +++
 tools/testing/selftests/arm64/Makefile        |   2 +-
 .../selftests/arm64/arm_cpumod/Makefile       |   5 +
 .../selftests/arm64/arm_cpumod/arm_cpumod.sh  | 543 ++++++++++++++++++
 10 files changed, 1262 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/arch/arm64/arm-cpumod.rst
 create mode 100644 arch/arm64/kernel/arm_cpumod.c
 create mode 100644 arch/arm64/kernel/arm_cpumod_internal.h
 create mode 100644 tools/testing/selftests/arm64/arm_cpumod/Makefile
 create mode 100755 tools/testing/selftests/arm64/arm_cpumod/arm_cpumod.sh

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2.43.0