Re: [PATCH bpf-next v2 2/2] bpf, mips: Add support for BPF_SDIV and BPF_SMOD

[Johan Almbladh]

On Wed, Jul 29, 2026 at 6:30 PM Nicholas Dudar <main.kalliope@xxxxxxxxx> wrote:
>
> The MIPS JITs handle BPF_DIV and BPF_MOD without inspecting
> insn->off, which distinguishes BPF_SDIV and BPF_SMOD. Signed
> operations therefore use unsigned instructions, or unsigned helpers
> for ALU64 on 32-bit MIPS, and produce unsigned results for negative
> operands.

Thanks for the patch! In addition to your SDIV/SMOD fixes, I would
like to bring the MIPS JIT to full v4 compliance. All the tests in
test_bpf should be jit'ed and pass. Let me know if you are already
working on the remaining v4 support, otherwise I could look into it.

> arch/mips/include/asm/uasm.h   |  6 ++++
> arch/mips/mm/uasm-mips.c       | 10 ++++++
> arch/mips/mm/uasm.c            | 20 ++++++++----

Consider making the uasm additions a separate patch. It would be more clear IMO.

> +static s64 jit_smod64(s64 a, s64 b)
> +{
> +       u64 quot = div64_s64(a, b);
> +
> +       return a - quot * b;

Should not "quot" be s64? It would be more clear to not rely on
implicit signed/unsigned promotions.

> @@ -650,6 +665,7 @@ int build_insn(const struct bpf_insn *insn, struct jit_context *ctx)
>         u8 code = insn->code;
>         s16 off = insn->off;
>         s32 imm = insn->imm;
> +       bool is_signed = off == 1;

The interpretation of the offset field is opcode-specific. The
SDIV/SMOD instructions are encoded as DIV/MOD with off = 1. MOVSX
instead use off to encode the width of the load.

Please propagate the raw offset value and do the interpretation in the
div/mod helpers instead. This also makes it easier to add the
remaining v4 instructions later.

Thanks,
Johan