netdev - Re: [RFC bpf-next PATCH] bpf: add comments to BPF ld/ldx sizes

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Date:   Wed, 17 Jan 2018 00:21:27 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: [RFC bpf-next PATCH] bpf: add comments to BPF ld/ldx sizes

On 01/16/2018 12:31 PM, Jesper Dangaard Brouer wrote:
> Doc BPF ld/ldx size defines, as it help me understand the code in filter.c.
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---
>  0 files changed
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 395d261948de..4729d9a002d4 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -17,7 +17,7 @@
>  #define BPF_ALU64	0x07	/* alu mode in double word width */
>  
>  /* ld/ldx fields */
> -#define BPF_DW		0x18	/* double word */
> +#define BPF_DW		0x18	/* double word (64-bit) */
>  #define BPF_XADD	0xc0	/* exclusive add */
>  
>  /* alu/jmp fields */
> diff --git a/include/uapi/linux/bpf_common.h b/include/uapi/linux/bpf_common.h
> index 18be90725ab0..ee97668bdadb 100644
> --- a/include/uapi/linux/bpf_common.h
> +++ b/include/uapi/linux/bpf_common.h
> @@ -15,9 +15,10 @@
>  
>  /* ld/ldx fields */
>  #define BPF_SIZE(code)  ((code) & 0x18)
> -#define		BPF_W		0x00
> -#define		BPF_H		0x08
> -#define		BPF_B		0x10
> +#define		BPF_W		0x00 /* 32-bit */
> +#define		BPF_H		0x08 /* 16-bit */
> +#define		BPF_B		0x10 /*  8-bit */
> +/* eBPF		BPF_DW		0x18    64-bit */

Hmm, I don't really mind, but we do have it documented in:

  Documentation/networking/filter.txt +942

Feels like if we put a comment only on BPF_{B,H,W}, then we
might also want to document all the others such as ALU ops,
etc.

>  #define BPF_MODE(code)  ((code) & 0xe0)
>  #define		BPF_IMM		0x00
>  #define		BPF_ABS		0x20
>