Re: [PATCH bpf-next v4 2/3] MIPS: uasm: Add signed div/mod and sign-extension emitters

[Nicholas Dudar]

> This isn't a bug, but would it read better to add the SEB and SEH emitters
> together with their first caller in the BPF_MOVSX series?

I grouped the native-MIPS uasm additions for SDIV/SMOD and MOVSX in one
preparatory patch, with instruction selection handled by their respective
JIT changes. MOVSX v3 will declare this series as a dependency. Keeping
SEB/SEH with their first callers would also be a valid split.

Until those callers arrive, the emitters are unused. Their definitions
do not generate instructions, and the existing opcode encodings are
preserved.

> and CONFIG_CPU_MICROMIPS kernels would hit build_insn()'s panic for them
> since arch/mips/mm/uasm-micromips.c isn't updated with their encodings.

That requires a microMIPS caller. Neither this series nor the existing
kernel code calls these emitters, and the planned MOVSX callers cannot
be built for microMIPS: MIPS only selects HAVE_EBPF_JIT when
!CPU_MICROMIPS.

I checked this on the posted v4: olddefconfig removes the attempted BPF
JIT settings with CPU_MICROMIPS enabled, and the microMIPS uasm object
builds successfully. Compiled-table comparisons also confirm that every
existing native and microMIPS opcode entry is preserved.

I also ran test_bpf on the official base and standalone v4, without
MOVSX, under QEMU Malta on little-endian MIPS32 R2 and MIPS64 R2.
Both showed exactly 14 signed DIV/MOD failures becoming passes, with
no other test-status changes.

Nicholas