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[v3,0/3] Libatomic: Add LSE128 atomics support for AArch64

Message ID 20240103012828.2446443-1-victor.donascimento@arm.com
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Series Libatomic: Add LSE128 atomics support for AArch64 | expand

Message

Victor Do Nascimento Jan. 3, 2024, 1:28 a.m. UTC 
v3 updates:

   1. In the absence of the `HWCAP_LSE128' feature bit in the current
   Linux Kernel release, the feature check continues to rely on a user
   space-issued `mrs' instruction.  Since the the ABI for exporting
   the AArch64 CPU ID/feature registers to userspace relies on
   FEAT_IDST [1], we make the ID_AA64ISAR0_EL1-mediated feature check
   contingent on having the HWCAP_CPUID bit set, ensuring FEAT_IDST
   support, avoiding potential runtime errors.

   2. It is established that, given LSE2 is mandatory from Armv8.4
   onward, LSE128 as introduced from Armv9.4 necessarily implies LSE2,
   such that a separate check for LSE2 is not necessary.

   3. Given that the infrastructure for exposing `mrs' to userspace
   hooks into the exception handler, the feature-register read is
   relatively expensive and ought to be avoided where possible.
   Consequently, where we can ascertain whether prerequisites are met
   via HWCAPS, we query these as a way of returning early where we
   known unequivocally that a given feature cannot be implemented due
   to unmet dependencies.  Such checks are added to both `has_lse2'
   and `has_lse128'.

Regression-tested on aarch64-none-linux-gnu on Cortex-A72 and
LSE128-enabled Armv-A Base RevC AEM FVP.

[1] https://www.kernel.org/doc/html/v6.6/arch/arm64/cpu-feature-registers.html

---

Building upon Wilco Dijkstra's work on AArch64 128-bit atomics for
Libatomic, namely the patches from [1] and [2],  this patch series
extends the library's  capabilities to dynamically select and emit
Armv9.4-a LSE128 implementations of atomic operations via ifuncs at
run-time whenever architectural support is present.

Regression tested on aarch64-linux-gnu target with LSE128-support.

[1] https://gcc.gnu.org/pipermail/gcc-patches/2023-June/620529.html
[2] https://gcc.gnu.org/pipermail/gcc-patches/2023-August/626358.html

Victor Do Nascimento (3):
  libatomic: atomic_16.S: Improve ENTRY, END and ALIAS macro interface
  libatomic: Enable LSE128 128-bit atomics for armv9.4-a
  aarch64: Add explicit checks for implicit LSE/LSE2 requirements.

 libatomic/Makefile.am                        |   3 +
 libatomic/Makefile.in                        |   1 +
 libatomic/acinclude.m4                       |  19 ++
 libatomic/auto-config.h.in                   |   3 +
 libatomic/config/linux/aarch64/atomic_16.S   | 251 ++++++++++++++++---
 libatomic/config/linux/aarch64/host-config.h |  36 ++-
 libatomic/configure                          |  59 ++++-
 libatomic/configure.ac                       |   1 +
 8 files changed, 331 insertions(+), 42 deletions(-)