netdev - Re: [PATCH bpf-next v4 2/3] bpf: implement map_gen

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Date:   Fri, 1 Nov 2019 00:48:04 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     netdev@...r.kernel.org, ast@...nel.org,
        jakub.kicinski@...ronome.com,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        bpf@...r.kernel.org, magnus.karlsson@...il.com,
        magnus.karlsson@...el.com, jonathan.lemon@...il.com,
        toke@...hat.com
Subject: Re: [PATCH bpf-next v4 2/3] bpf: implement map_gen_lookup() callback
 for XSKMAP

On Thu, Oct 31, 2019 at 09:47:48AM +0100, Björn Töpel wrote:
> From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> 
> Inline the xsk_map_lookup_elem() via implementing the map_gen_lookup()
> callback. This results in emitting the bpf instructions in place of
> bpf_map_lookup_elem() helper call and better performance of bpf
> programs.
> 
> Acked-by: Jonathan Lemon <jonathan.lemon@...il.com>
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> ---
>  kernel/bpf/xskmap.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/kernel/bpf/xskmap.c b/kernel/bpf/xskmap.c
> index edcbd863650e..fa32f775b4de 100644
> --- a/kernel/bpf/xskmap.c
> +++ b/kernel/bpf/xskmap.c
> @@ -163,6 +163,22 @@ struct xdp_sock *__xsk_map_lookup_elem(struct bpf_map *map, u32 key)
>  	return xs;
>  }
>  
> +static u32 xsk_map_gen_lookup(struct bpf_map *map, struct bpf_insn *insn_buf)
> +{
> +	const int ret = BPF_REG_0, mp = BPF_REG_1, index = BPF_REG_2;
> +	struct bpf_insn *insn = insn_buf;
> +
> +	*insn++ = BPF_LDX_MEM(BPF_W, ret, index, 0);
> +	*insn++ = BPF_JMP_IMM(BPF_JGE, ret, map->max_entries, 5);
> +	*insn++ = BPF_ALU64_IMM(BPF_LSH, ret, ilog2(sizeof(struct xsk_sock *)));
> +	*insn++ = BPF_ALU64_IMM(BPF_ADD, mp, offsetof(struct xsk_map, xsk_map));
> +	*insn++ = BPF_ALU64_REG(BPF_ADD, ret, mp);
> +	*insn++ = BPF_LDX_MEM(BPF_DW, ret, ret, 0);

Your map slots are always exactly sizeof(struct xdp_sock *), right? Wouldn't
this BPF_DW crash on 32 bit?

Meaning, it would have to be BPF_LDX_MEM(BPF_SIZEOF(struct xsk_sock *), ...)?

> +	*insn++ = BPF_JMP_IMM(BPF_JA, 0, 0, 1);
> +	*insn++ = BPF_MOV64_IMM(ret, 0);
> +	return insn - insn_buf;
> +}
> +
>  int __xsk_map_redirect(struct bpf_map *map, struct xdp_buff *xdp,
>  		       struct xdp_sock *xs)
>  {
> @@ -303,6 +319,7 @@ const struct bpf_map_ops xsk_map_ops = {
>  	.map_free = xsk_map_free,
>  	.map_get_next_key = xsk_map_get_next_key,
>  	.map_lookup_elem = xsk_map_lookup_elem,
> +	.map_gen_lookup = xsk_map_gen_lookup,
>  	.map_lookup_elem_sys_only = xsk_map_lookup_elem_sys_only,
>  	.map_update_elem = xsk_map_update_elem,
>  	.map_delete_elem = xsk_map_delete_elem,
> -- 
> 2.20.1
>