netdev - Re: [PATCH bpf-next v2 5/9] riscv, bpf: optimize BPF tail calls

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Message-ID: <mhng-01fe07e5-a27a-4fde-a7ab-22bbad6eb668@palmerdabbelt-glaptop>
Date:   Mon, 23 Dec 2019 10:29:39 -0800 (PST)
From:   Palmer Dabbelt <palmerdabbelt@...gle.com>
To:     Bjorn Topel <bjorn.topel@...il.com>
CC:     daniel@...earbox.net, ast@...nel.org, netdev@...r.kernel.org,
        Bjorn Topel <bjorn.topel@...il.com>,
        linux-riscv@...ts.infradead.org, bpf@...r.kernel.org
Subject:     Re: [PATCH bpf-next v2 5/9] riscv, bpf: optimize BPF tail calls

On Mon, 16 Dec 2019 01:13:39 PST (-0800), Bjorn Topel wrote:
> Remove one addi, and instead use the offset part of jalr.
>
> Signed-off-by: Björn Töpel <bjorn.topel@...il.com>
> ---
>  arch/riscv/net/bpf_jit_comp.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/arch/riscv/net/bpf_jit_comp.c b/arch/riscv/net/bpf_jit_comp.c
> index 2fc0f24ad30f..8aa19c846881 100644
> --- a/arch/riscv/net/bpf_jit_comp.c
> +++ b/arch/riscv/net/bpf_jit_comp.c
> @@ -552,7 +552,7 @@ static int epilogue_offset(struct rv_jit_context *ctx)
>  	return (to - from) << 2;
>  }
>
> -static void __build_epilogue(u8 reg, struct rv_jit_context *ctx)
> +static void __build_epilogue(bool is_tail_call, struct rv_jit_context *ctx)
>  {
>  	int stack_adjust = ctx->stack_size, store_offset = stack_adjust - 8;
>
> @@ -589,9 +589,11 @@ static void __build_epilogue(u8 reg, struct rv_jit_context *ctx)
>
>  	emit(rv_addi(RV_REG_SP, RV_REG_SP, stack_adjust), ctx);
>  	/* Set return value. */
> -	if (reg == RV_REG_RA)
> +	if (!is_tail_call)
>  		emit(rv_addi(RV_REG_A0, RV_REG_A5, 0), ctx);
> -	emit(rv_jalr(RV_REG_ZERO, reg, 0), ctx);
> +	emit(rv_jalr(RV_REG_ZERO, is_tail_call ? RV_REG_T3 : RV_REG_RA,
> +		     is_tail_call ? 4 : 0), /* skip TCC init */
> +	     ctx);
>  }
>
>  /* return -1 or inverted cond */
> @@ -751,9 +753,8 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
>  	if (is_12b_check(off, insn))
>  		return -1;
>  	emit(rv_ld(RV_REG_T3, off, RV_REG_T2), ctx);
> -	emit(rv_addi(RV_REG_T3, RV_REG_T3, 4), ctx);
>  	emit(rv_addi(RV_REG_TCC, RV_REG_T1, 0), ctx);
> -	__build_epilogue(RV_REG_T3, ctx);
> +	__build_epilogue(true, ctx);
>  	return 0;
>  }
>
> @@ -1504,7 +1505,7 @@ static void build_prologue(struct rv_jit_context *ctx)
>
>  static void build_epilogue(struct rv_jit_context *ctx)
>  {
> -	__build_epilogue(RV_REG_RA, ctx);
> +	__build_epilogue(false, ctx);
>  }
>
>  static int build_body(struct rv_jit_context *ctx, bool extra_pass)

Reviewed-by: Palmer Dabbelt <palmerdabbelt@...gle.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@...gle.com>