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

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Date:	Wed, 11 Jun 2014 00:13:38 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	ast@...mgrid.com
Cc:	chema@...gle.com, edumazet@...gle.com, dborkman@...hat.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: filter: cleanup A/X name usage

From: Alexei Starovoitov <ast@...mgrid.com>
Date: Fri,  6 Jun 2014 14:46:06 -0700

> 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>

Applied.
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