linux-kernel - Re: [PATCH RFC v4 net-next 01/26] net: filter: add "load 64-bit immediate" eBPF instruction

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Message-ID: <53EBA2EC.3030306@redhat.com>
Date:	Wed, 13 Aug 2014 19:39:56 +0200
From:	Daniel Borkmann <dborkman@...hat.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Chema Gonzalez <chema@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v4 net-next 01/26] net: filter: add "load 64-bit immediate"
 eBPF instruction

On 08/13/2014 07:34 PM, Alexei Starovoitov wrote:
> On Wed, Aug 13, 2014 at 2:17 AM, Daniel Borkmann <dborkman@...hat.com> wrote:
>> On 08/13/2014 09:57 AM, Alexei Starovoitov wrote:
>>>
>>> add BPF_LD_IMM64 instruction to load 64-bit immediate value into register.
>>> All previous instructions were 8-byte. This is first 16-byte instruction.
>>> Two consecutive 'struct bpf_insn' blocks are interpreted as single
>>> instruction:
>>> insn[0/1].code = BPF_LD | BPF_DW | BPF_IMM
>>> insn[0/1].dst_reg = destination register
>>> insn[0].imm = lower 32-bit
>>> insn[1].imm = upper 32-bit
>>>
>>> Classic BPF has similar instruction: BPF_LD | BPF_W | BPF_IMM
>>> which loads 32-bit immediate value into a register.
>>>
>>> x64 JITs it as single 'movabsq %rax, imm64'
>>> arm64 may JIT as sequence of four 'movk x0, #imm16, lsl #shift' insn
>>>
>>> Note that old eBPF programs are binary compatible with new interpreter.
>>>
>>> Signed-off-by: Alexei Starovoitov <ast@...mgrid.com>
>>
>>
>> For follow-ups on this series, can you put the actual motivation
>> for this change from the cover letter into this commit log as it
>> otherwise doesn't say anything clearly why it is needed. Code and
>> test case looks good to me.
>
> ok. As you saw the full explanation is long, so I opted for 'it_does_this'
> commit log. In the next rev will add more reasons to this log. Sure.

Great, thanks.
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