netdev - Re: [PATCH net-next] net: filter: cleanup A/X name usage

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Date:	Sun, 08 Jun 2014 22:21:31 +0200
From:	Daniel Borkmann <dborkman@...hat.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	Chema Gonzalez <chema@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: filter: cleanup A/X name usage

On 06/06/2014 11:46 PM, Alexei Starovoitov wrote:
> The macro 'A' used in internal BPF interpreter:
>   #define A regs[insn->a_reg]
> was easily confused with the name of classic BPF register 'A', since
> 'A' would mean two different things depending on context.
>
> This patch is trying to clean up the naming and clarify its usage in the
> following way:
>
> - A and X are names of two classic BPF registers
>
> - BPF_REG_A denotes internal BPF register R0 used to map classic register A
>    in internal BPF programs generated from classic
>
> - BPF_REG_X denotes internal BPF register R7 used to map classic register X
>    in internal BPF programs generated from classic
>
> - internal BPF instruction format:
> struct sock_filter_int {
>          __u8    code;           /* opcode */
>          __u8    dst_reg:4;      /* dest register */
>          __u8    src_reg:4;      /* source register */
>          __s16   off;            /* signed offset */
>          __s32   imm;            /* signed immediate constant */
> };
>
> - BPF_X/BPF_K is 1 bit used to encode source operand of instruction
> In classic:
>    BPF_X - means use register X as source operand
>    BPF_K - means use 32-bit immediate as source operand
> In internal:
>    BPF_X - means use 'src_reg' register as source operand
>    BPF_K - means use 32-bit immediate as source operand
>
> Suggested-by: Chema Gonzalez <chema@...gle.com>
> Signed-off-by: Alexei Starovoitov <ast@...mgrid.com>

Acked-by: Daniel Borkmann <dborkman@...hat.com>

You could also have mentioned in the changelog that you replace
direct access to ctx pointer with CTX register, it's valid since
we emit a mov from ARG1 to CTX at the beginning which contains
ctx. Anyway, looks good to me.
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