On Sat, Sep 3, 2022 at 6:11 PM Tiezhu Yang <yangtiezhu@xxxxxxxxxxx> wrote:
>
>
>
> On 09/03/2022 04:32 PM, Huacai Chen wrote:
> > On Thu, Sep 1, 2022 at 10:27 AM Tiezhu Yang <yangtiezhu@xxxxxxxxxxx> wrote:
> >>
> >> BPF programs are normally handled by a BPF interpreter, add BPF JIT
> >> support for LoongArch to allow the kernel to generate native code
> >> when a program is loaded into the kernel, this will significantly
> >> speed-up processing of BPF programs.
>
> [...]
>
> >> +
> >> +static inline int emit_cond_jmp(struct jit_ctx *ctx, u8 cond, enum loongarch_gpr rj,
> >> + enum loongarch_gpr rd, int jmp_offset)
> >> +{
> >> + /*
> >> + * A large PC-relative jump offset may overflow the immediate field of
> >> + * the native conditional branch instruction, triggering a conversion
> >> + * to use an absolute jump instead, this jump sequence is particularly
> >> + * nasty. For now, use cond_jmp_offs26() directly to keep it simple.
> >> + * In the future, maybe we can add support for far branching, the branch
> >> + * relaxation requires more than two passes to converge, the code seems
> >> + * too complex to understand, not quite sure whether it is necessary and
> >> + * worth the extra pain. Anyway, just leave it as it is to enhance code
> >> + * readability now.
> > Oh, no. I don't think this is a very difficult problem, because the
> > old version has already solved [1]. Please improve your code and send
> > V4.
> > BTW, I have committed V3 with some small modifications in
> > https://github.com/loongson/linux/commits/loongarch-next, please make
> > V4 based on that.
> >
> > [1] https://github.com/loongson/linux/commit/e20b2353f40cd13720996524e1df6d0ca086eeb8#diff-6d2f4f5a862a5dce12f8eb0feeca095825c4ed1c2e7151b0905fb8d03c98922e
> >
> > --------code in the old version--------
> > static inline void emit_cond_jump(struct jit_ctx *ctx, u8 cond, enum
> > loongarch_gpr rj,
> > enum loongarch_gpr rd, int jmp_offset)
> > {
> > if (is_signed_imm16(jmp_offset))
> > cond_jump_offs16(ctx, cond, rj, rd, jmp_offset);
> > else if (is_signed_imm26(jmp_offset))
> > cond_jump_offs26(ctx, cond, rj, rd, jmp_offset);
> > else
> > cond_jump_offs32(ctx, cond, rj, rd, jmp_offset);
> > }
> >
> > static inline void emit_uncond_jump(struct jit_ctx *ctx, int
> > jmp_offset, bool is_exit)
> > {
> > if (is_signed_imm26(jmp_offset))
> > uncond_jump_offs26(ctx, jmp_offset);
> > else
> > uncond_jump_offs32(ctx, jmp_offset, is_exit);
> > }
> > --------end of code--------
> >
> > Huacai
> >
>
> Hi Huacai,
>
> This change is to pass the special test cases:
> "a new type of jump test where the program jumps forwards
> and backwards with increasing offset. It mainly tests JITs where a
> relative jump may generate different JITed code depending on the offset
> size."
>
> They are introduced in commit a7d2e752e520 ("bpf/tests: Add staggered
> JMP and JMP32 tests") after the old internal version you mentioned.
>
> Here, we use the native instructions to enlarge the jump range to 26 bit
> directly rather than 16 bit first, and also take no account of more than
> 26 bit case because there is no native instruction and it needs to emulate.
>
> As the code comment said, this is to enhance code readability now.
I'm not familiar with bpf, Daniel, Ruoyao and Xuerui, what do you
think about it?
Huacai
>
> Thanks,
> Tiezhu
>
