linux-toradex.git/arch/arm64/include/asm/lsui.h, branch master Linux kernel for Apalis and Colibri modules arm64: futex: Support futex with FEAT_LSUI 2026-03-27T12:52:04+00:00 Yeoreum Yun yeoreum.yun@arm.com 2026-03-14T17:51:30+00:00 44adf2bf40efe56c68c1563779ab2047eb0a57ea Current futex atomic operations are implemented using LL/SC instructions while temporarily clearing PSTATE.PAN and setting PSTATE.TCO (if KASAN_HW_TAGS is enabled). With Armv9.6, FEAT_LSUI provides atomic instructions for user memory access in the kernel without the need for PSTATE bits toggling. Use the FEAT_LSUI instructions to implement the futex atomic operations. Note that some futex operations do not have a matching LSUI instruction, (eor or word-sized cmpxchg). For such cases, use cas{al}t to implement the operation. Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com> [catalin.marinas@arm.com: add comment on -EAGAIN in __lsui_futex_cmpxchg()] Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> 
Current futex atomic operations are implemented using LL/SC instructions
while temporarily clearing PSTATE.PAN and setting PSTATE.TCO (if
KASAN_HW_TAGS is enabled). With Armv9.6, FEAT_LSUI provides atomic
instructions for user memory access in the kernel without the need for
PSTATE bits toggling.

Use the FEAT_LSUI instructions to implement the futex atomic operations.
Note that some futex operations do not have a matching LSUI instruction,
(eor or word-sized cmpxchg). For such cases, use cas{al}t to implement
the operation.

Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
[catalin.marinas@arm.com: add comment on -EAGAIN in __lsui_futex_cmpxchg()]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>