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Message-ID: <68ec5a7f3cc63dc19397b3ce0649716e0fac8d49.camel@kylinos.cn>
Date: Fri, 30 May 2025 09:21:54 +0800
From: jianghaoran <jianghaoran@...inos.cn>
To: Hengqi Chen <hengqi.chen@...il.com>
Cc: loongarch@...ts.linux.dev, bpf@...r.kernel.org,
linux-kernel@...r.kernel.org, kernel@...0n.name, chenhuacai@...nel.org,
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: 回复：[PATCH] LoongArch: BPF: Optimize
the calculation method of jmp_offset in the emit_bpf_tail_call function
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.
>
> [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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