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Message-ID: <CAAhV-H5NrGb9ofaKdqUQ3Qc6RK3c=Ngy6KsxX2GaOqUb0SQRdw@mail.gmail.com>
Date: Tue, 24 Jun 2025 20:09:01 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: jianghaoran <jianghaoran@...inos.cn>
Cc: Hengqi Chen <hengqi.chen@...il.com>, loongarch@...ts.linux.dev, bpf@...r.kernel.org,
linux-kernel@...r.kernel.org, kernel@...0n.name, yangtiezhu@...ngson.cn,
haoluo@...gle.com, jolsa@...nel.org, sdf@...ichev.me, kpsingh@...nel.org,
john.fastabend@...il.com, yonghong.song@...ux.dev, song@...nel.org,
eddyz87@...il.com, martin.lau@...ux.dev, andrii@...nel.org,
daniel@...earbox.net
Subject: Re: [PATCH] LoongArch: BPF: Optimize the calculation method of
jmp_offset in the emit_bpf_tail_call function
Hi, Haoran,
On Fri, May 30, 2025 at 9:22 AM jianghaoran <jianghaoran@...inos.cn> wrote:
>
>
>
>
>
> 在 2025-05-29星期四的 10:02 +0800，Hengqi Chen写道：
> > Hi Haoran,
> >
> > On Wed, May 28, 2025 at 6:40 PM Haoran Jiang <
> > jianghaoran@...inos.cn
> > > wrote:
> > > For a ebpf subprog JIT，the last call bpf_int_jit_compile
> > > function will
> > > directly enter the skip_init_ctx process. At this point,
> > > out_offset = -1,
> > > the jmp_offset in emit_bpf_tail_call is calculated
> > > by #define jmp_offset (out_offset - (cur_offset)) is a negative
> > > number,
> > > which does not meet expectations.The final generated assembly
> > > as follow.
> > >
> > > 54: bgeu $a2, $t1, -8 # 0x0000004c
> > > 58: addi.d $a6, $s5, -1
> > > 5c: bltz $a6, -16 # 0x0000004c
> > > 60: alsl.d $t2, $a2, $a1, 0x3
> > > 64: ld.d $t2, $t2, 264
> > > 68: beq $t2, $zero, -28 # 0x0000004c
> > >
> > > Before apply this patch, the follow test case will reveal soft
> > > lock issues.
> > >
> > > cd tools/testing/selftests/bpf/
> > > ./test_progs --allow=tailcalls/tailcall_bpf2bpf_1
> > >
> > > dmesg:
> > > watchdog: BUG: soft lockup - CPU#2 stuck for 26s!
> > > [test_progs:25056]
> > >
> >
> > This is a known issue. Does this change pass all tailcall tests ?
> > If not, please refer to the tailcall hierarchy patchset([1]).
> > We should address it once and for all. Thanks.
Do you mean you will update this patch?
Huacai
> >
> > [1]:
> > https://lore.kernel.org/bpf/20240714123902.32305-1-hffilwlqm@gmail.com/
> >
> > Thanks,I'll keep looking into these patches.
> > > Signed-off-by: Haoran Jiang <
> > > jianghaoran@...inos.cn
> > > >
> > > ---
> > > arch/loongarch/net/bpf_jit.c | 28 +++++++++-------------------
> > > 1 file changed, 9 insertions(+), 19 deletions(-)
> > >
> > > diff --git a/arch/loongarch/net/bpf_jit.c
> > > b/arch/loongarch/net/bpf_jit.c
> > > index fa1500d4aa3e..d85490e7de89 100644
> > > --- a/arch/loongarch/net/bpf_jit.c
> > > +++ b/arch/loongarch/net/bpf_jit.c
> > > @@ -208,9 +208,7 @@ bool bpf_jit_supports_far_kfunc_call(void)
> > > return true;
> > > }
> > >
> > > -/* initialized on the first pass of build_body() */
> > > -static int out_offset = -1;
> > > -static int emit_bpf_tail_call(struct jit_ctx *ctx)
> > > +static int emit_bpf_tail_call(int insn, struct jit_ctx *ctx)
> > > {
> > > int off;
> > > u8 tcc = tail_call_reg(ctx);
> > > @@ -220,9 +218,8 @@ static int emit_bpf_tail_call(struct
> > > jit_ctx *ctx)
> > > u8 t2 = LOONGARCH_GPR_T2;
> > > u8 t3 = LOONGARCH_GPR_T3;
> > > const int idx0 = ctx->idx;
> > > -
> > > -#define cur_offset (ctx->idx - idx0)
> > > -#define jmp_offset (out_offset - (cur_offset))
> > > + int tc_ninsn = 0;
> > > + int jmp_offset = 0;
> > >
> > > /*
> > > * a0: &ctx
> > > @@ -232,8 +229,11 @@ static int emit_bpf_tail_call(struct
> > > jit_ctx *ctx)
> > > * if (index >= array->map.max_entries)
> > > * goto out;
> > > */
> > > + tc_ninsn = insn ? ctx->offset[insn+1] - ctx-
> > > >offset[insn] :
> > > + ctx->offset[0];
> > > off = offsetof(struct bpf_array, map.max_entries);
> > > emit_insn(ctx, ldwu, t1, a1, off);
> > > + jmp_offset = tc_ninsn - (ctx->idx - idx0);
> > > /* bgeu $a2, $t1, jmp_offset */
> > > if (emit_tailcall_jmp(ctx, BPF_JGE, a2, t1, jmp_offset)
> > > < 0)
> > > goto toofar;
> > > @@ -243,6 +243,7 @@ static int emit_bpf_tail_call(struct
> > > jit_ctx *ctx)
> > > * goto out;
> > > */
> > > emit_insn(ctx, addid, REG_TCC, tcc, -1);
> > > + jmp_offset = tc_ninsn - (ctx->idx - idx0);
> > > if (emit_tailcall_jmp(ctx, BPF_JSLT, REG_TCC,
> > > LOONGARCH_GPR_ZERO, jmp_offset) < 0)
> > > goto toofar;
> > >
> > > @@ -254,6 +255,7 @@ static int emit_bpf_tail_call(struct
> > > jit_ctx *ctx)
> > > emit_insn(ctx, alsld, t2, a2, a1, 2);
> > > off = offsetof(struct bpf_array, ptrs);
> > > emit_insn(ctx, ldd, t2, t2, off);
> > > + jmp_offset = tc_ninsn - (ctx->idx - idx0);
> > > /* beq $t2, $zero, jmp_offset */
> > > if (emit_tailcall_jmp(ctx, BPF_JEQ, t2,
> > > LOONGARCH_GPR_ZERO, jmp_offset) < 0)
> > > goto toofar;
> > > @@ -263,22 +265,11 @@ static int emit_bpf_tail_call(struct
> > > jit_ctx *ctx)
> > > emit_insn(ctx, ldd, t3, t2, off);
> > > __build_epilogue(ctx, true);
> > >
> > > - /* out: */
> > > - if (out_offset == -1)
> > > - out_offset = cur_offset;
> > > - if (cur_offset != out_offset) {
> > > - pr_err_once("tail_call out_offset = %d,
> > > expected %d!\n",
> > > - cur_offset, out_offset);
> > > - return -1;
> > > - }
> > > -
> > > return 0;
> > >
> > > toofar:
> > > pr_info_once("tail_call: jump too far\n");
> > > return -1;
> > > -#undef cur_offset
> > > -#undef jmp_offset
> > > }
> > >
> > > static void emit_atomic(const struct bpf_insn *insn, struct
> > > jit_ctx *ctx)
> > > @@ -916,7 +907,7 @@ static int build_insn(const struct bpf_insn
> > > *insn, struct jit_ctx *ctx, bool ext
> > > /* tail call */
> > > case BPF_JMP | BPF_TAIL_CALL:
> > > mark_tail_call(ctx);
> > > - if (emit_bpf_tail_call(ctx) < 0)
> > > + if (emit_bpf_tail_call(i, ctx) < 0)
> > > return -EINVAL;
> > > break;
> > >
> > > @@ -1342,7 +1333,6 @@ struct bpf_prog
> > > *bpf_int_jit_compile(struct bpf_prog *prog)
> > > if (tmp_blinded)
> > > bpf_jit_prog_release_other(prog, prog ==
> > > orig_prog ? tmp : orig_prog);
> > >
> > > - out_offset = -1;
> > >
> > > return prog;
> > >
> > > --
> > > 2.43.0
> > >
>
>
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