[PATCH 0/3] sparc32: SMP futexes, casa and idiv emulation

[Imre Kaloz <kaloz@kernel.org>]

sun4m has no compare-and-swap instruction and casa is optional on
LEON, so SMP sparc32 has had no futexes, and user code on those CPUs
has no CAS to build locks from.

Patch 1 implements the futex atomic ops in atomic32.c and drops the
!(SPARC32 && SMP) dependency of FUTEX. A LEON part that implements
casa, found by a boot-time probe, runs casa on the user word; other
CPUs take the __atomic_hash spinlocks.

Patch 2 completes user casa from illegal_instruction on CPUs without
it, through patch 1's futex_atomic_cmpxchg_inatomic(), so emulated
casa and the futex ops serialize on the same lock. It is atomic
against itself and against the futex ops, not against a plain store
to the same word, so a C library using it has to do its atomic stores
with casa too.

Patch 3 completes udiv, sdiv and their -cc forms from the same
do_illegal_instruction() dispatch: SuperSPARC and SuperSPARC-II trap
those when the {Y, rs1} dividend has significant bits above bit 51,
which gcc -mcpu=v8 output, including libgcc's __udivdi3, can produce.
It shares patch 2's windowed-operand helpers and the dispatch block,
which is why it comes last.

Patch 1 comes first because patch 2 is built on it. On its own it
races nothing: without patch 2, user casa on these CPUs is SIGILL.

Magnus Lindholm has an independent RFC covering the same ground from
a different ABI: "[RFC PATCH 0/5] sparc32: kernel assisted
compare-and-swap, and futex on SMP" (patchwork sparclinux, Message-Id
20260923201830.865553-1-linmag7@gmail.com), with a companion glibc
series (patchwork glibc, Message-Id
20260924064228.867909-1-linmag7@gmail.com). That RFC adds a new
software trap, ta 0x11 (trap type 0x91), extending David Miller's
2016 CAS-trap prototype at ta 0x23 rather than completing it, so
userspace has to be rebuilt to call it. This series instead
completes the casa opcode itself, so a binary already emitting casa
(gcc -mcpu=leon3 or -mcpu=v9 output, or a C library that encodes it
as a .word) runs unmodified on a CPU without hardware casa, with no
new trap number and no libc rebuild. Happy to share the futex and
do_illegal_instruction() plumbing with whichever ABI lands.

Andreas: do LEON3FT parts want the GRLIB-TN-0010/0011 "nop; .balignl
16" alignment before the casa word here, or is the plain encoding as
posted fine for them?

Notes, not for the log:

- checkpatch on patch 1: "Lines should not end with a '('" is the
  __asm__ __volatile__( idiom of uaccess_32.h; the -ENOSYS warning is
  the futex API's return for an unknown op, as on every architecture.
  "does MAINTAINERS need updating?" on patches 2 and 3 is covered by
  the F: arch/sparc/ entry.
- On qemu's SS-20 with two SuperSPARCs and the casa emulation alone,
  futex() returns ENOSYS and musl's mutex, PI mutex and robust mutex
  tests fail. With patches 1 and 2 they pass, as does a PI lock
  contended between user casa and FUTEX_LOCK_PI/FUTEX_UNLOCK_PI on
  both CPUs.
- The casa branch of the futex ops has not run on LEON hardware. The
  probe runs on LEON only; its fixup path, which a LEON without casa
  takes, has run only in a test kernel that forced the probe on
  qemu's SS-20.
- The divide emulation has run on no CPU that traps it. No sparc32
  model in qemu raises the trap, and no SuperSPARC or SuperSPARC-II
  was available for this series; it is exercised only by build and
  link.

Imre Kaloz (3):
  sparc32: support futexes on SMP
  sparc32: emulate casa on V8 CPUs
  sparc32: emulate integer divide taken as illegal_instruction

 arch/sparc/include/asm/futex_32.h |   6 +-
 arch/sparc/kernel/Makefile        |   2 +
 arch/sparc/kernel/cas_emu_32.c    | 117 +++++++++++++++++++++++++++
 arch/sparc/kernel/div_emu_32.c    | 127 ++++++++++++++++++++++++++++++
 arch/sparc/kernel/entry.h         |   2 +
 arch/sparc/kernel/traps_32.c      |  23 +++++-
 arch/sparc/kernel/unimp_32.h      |  71 +++++++++++++++++
 arch/sparc/lib/atomic32.c         | 122 ++++++++++++++++++++++++++++
 init/Kconfig                      |   1 -
 9 files changed, 468 insertions(+), 3 deletions(-)
 create mode 100644 arch/sparc/kernel/cas_emu_32.c
 create mode 100644 arch/sparc/kernel/div_emu_32.c
 create mode 100644 arch/sparc/kernel/unimp_32.h


base-commit: 62f4c998b297cf233997a2b4cd6fc2d2df0319c9
-- 
2.47.3