[RFC bpf-next 0/2] bpf, mips: Add BPF_MEMSX support to the JITs

[Nicholas Dudar <main.kalliope@gmail.com>]

bpf-next is closed for the merge window, so I am sending this for
review. I plan to post this series after it reopens.

The MIPS32 and MIPS64 JITs reject BPF_MEMSX byte, half-word, and word
instructions, so a program containing one cannot run when JIT execution
is required.

First factor ordinary narrow-load selection into backend-local helpers.
Then extend those helpers to select signed native loads for BPF_MEMSX.
MIPS32 observes the load-delay rule and propagates the low-word sign into
the high half; MIPS64 obtains the full result from the native load.
Ordinary loads remain unsigned, and BPF_MEMSX with BPF_DW remains
unsupported.

This series must be applied after the in-review MIPS MOVSX and SDIV/SMOD
series:

MOVSX: https://lore.kernel.org/bpf/20260819010523.1057789-1-main.kalliope@gmail.com/
SDIV/SMOD: https://lore.kernel.org/bpf/20260810194215.3754591-1-main.kalliope@gmail.com/

Applying MEMSX removes a whole-program JIT fallback that otherwise keeps
mixed programs containing unsupported MOVSX or signed DIV/MOD out of the
JIT.

The ordering discussion is here:

https://lore.kernel.org/bpf/CAJZwKkis=3NGw9At0WfiZaRYEoMPbQfqJKN0e+vtRDvp4VY5Ng@mail.gmail.com/

On four little-endian profiles, patch 1 leaves the complete test_bpf
results with 1031 passed/31 failed on MIPS32 and 1030/31 on MIPS64.
Patch 2 changes the three MEMSX cases, yielding 1034/28 and 1033/28,
respectively. Ordinary unsigned-load controls passed at each boundary.

Selector-focused QEMU testing covered MIPS32 base, R2, and R6 plus
pre-R6 MIPS64, each in big- and little-endian configurations. The focused
mixed-operation matrix reproduced the unsafe MEMSX-first ordering and the
correct fully composed result on MIPS32 and MIPS64, both big- and
little-endian. No physical MIPS hardware was tested.

Nicholas Dudar (2):
  bpf, mips: Factor narrow loads out of emit_ldx()
  bpf, mips: Add BPF_MEMSX support to the JITs

 arch/mips/net/bpf_jit_comp32.c | 51 ++++++++++++++++++++++++++++------
 arch/mips/net/bpf_jit_comp64.c | 47 +++++++++++++++++++++++++++----
 2 files changed, 85 insertions(+), 13 deletions(-)


base-commit: 91ec2035134982b98fab0609a9fd8480e8217dc1
prerequisite-patch-id: aa8ca3f50fc4b0160c5fdba43a92e4b7ec3f2e07
prerequisite-patch-id: bda4c3f3d59b48d784ed98bd8558d9591399bc86
prerequisite-patch-id: 35236f563c0536859077709d52b345b696e5c26e
prerequisite-patch-id: c0f282b0e57ecd60e9582fb1e89e4b3112ae7893
prerequisite-patch-id: 4e93eed38f6585e21fc13931c314907b4e437ef5