netdev - Re: [PATCH net-next v3 8/9] net: filter: rework/optimize internal BPF interpreter's instruction set

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Date:	Thu, 27 Mar 2014 13:30:47 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	David Miller <davem@...emloft.net>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	Network Development <netdev@...r.kernel.org>,
	Hagen Paul Pfeifer <hagen@...u.net>,
	Kees Cook <keescook@...omium.org>, pmoore@...hat.com,
	Ingo Molnar <mingo@...nel.org>, hpa@...ux.intel.com,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v3 8/9] net: filter: rework/optimize internal BPF
 interpreter's instruction set

On Thu, Mar 27, 2014 at 12:23 PM, David Miller <davem@...emloft.net> wrote:
> From: Daniel Borkmann <dborkman@...hat.com>
> Date: Wed, 26 Mar 2014 22:06:09 +0100
>
>>   - Adds swab insns for 32/64-bit
>
> I don't like this.
>
> You don't want a swab instruction, you want "endian X to endian Y".  Just
> like we have "cpu_to_le32()", "le32_to_cpu()" et al. in the kernel.
>
> That way the user can be completely oblivious as to the endianness of
> the cpu it's running on.

Agreed.
If we want to expose ebpf instruction set to the userspace as-is then
it completely makes sense.

The reason we picked swab style is two fold:
1. swab32/64 maps as-is to native cpu instructions during jit.
2. assumption was that new_socket_filters/tracing_filters/ovs_filters
   will be written in C and will use standard macro ntohl() that
   will be expanded to __builtin_swap or nop by gcc/llvm,
  so users don't need to worry about underlying endianness while
  writing filters in C.

> So if you ask for a "to little endian" swab, if the chip is
> little-endian then no code needs to be emitted at all, it's a nop.
>
> There is zero reason for the BPF program emitted by userspace to be
> dependant upon the cpu endianness.

Agree.
with cpu_to_le and cpu_to_be instructions the programs written in C
and written directly in ebpf won't need to worry about endianness.
Best of both.

We'll fix and resend.

Thanks
Alexei
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