netdev - Re: [RFC PATCH 0/1] BPF JIT for PPC64

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Message-Id: <750BF3E9-A788-4302-8AA1-4630614E20DC@kernel.crashing.org>
Date:	Fri, 24 Jun 2011 04:16:26 -0500
From:	Kumar Gala <galak@...nel.crashing.org>
To:	Matt Evans <matt@...abs.org>
Cc:	netdev@...r.kernel.org,
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [RFC PATCH 0/1] BPF JIT for PPC64


On Jun 24, 2011, at 1:02 AM, Matt Evans wrote:

> Hi,
> 
> 
> Inspired by Eric Dumazet's x86-64 compiler for Berkeley Packet Filter programs,
> I've written a BPF compiler for 64-bit PowerPC.  Although it hasn't finished its
> strenuous testing regime, I'll have intermittent net access for a couple of
> weeks so thought I'd post it for feedback now and submit a 'proper' version when
> I'm back.
> 
> It's a fairly simple code generator, following a similar structure to the x86
> version.  The filter programs are an array of opcode/constant/branch destination
> structs, and can perform arithmetic/logical/comparison operations on two virtual
> registers A and X, loads from packet headers/data and accesses to local
> variables, M[].  Branching is also supported, but only forwards and only within
> the extent of the program.
> 
> I would probably describe this as more of a "static template binary translator"
> than a "JIT" but have kept naming consistent :)
> 
> 
> Features include:
> 
> - Filter code is generated as an ABI-compliant function, stackframe &
>  prologue/epilogue if necessary.
> 
> - Simple filters (e.g. RET nn) need no stackframe or save/restore code so
>  generate into only an li/blr.
> 
> - Local variables, M[], live in registers
> 
> - I believe this supports all BPF opcodes, although "complicated" loads from
>  negative packet offsets (e.g. SKF_LL_OFF) are not yet supported.
> 
> Caveats include:  :)
> 
> - Packet data loads call out to simple helper functions (bpf_jit.S) which
>  themselves may fall back to a trampoline to skb_copy_bits.  I haven't decided
>  whether (as per comments there) it would be better to generate the simple
>  loads inline and only call out in the slow case.
> 
> - Branches currently generate to "bcc 1f; b <far dest>; 1:" or
>  "bcc <near dest> ; nop" so either case is the same size.  Multiple passes of
>  assembly are used (the first gets an idea of how big everything is and what
>  features are required), the next generates everything at accurate size, the
>  third generates everything with accurate branch destination addresses); I
>  intend not to nop-pad the short branch case but changing code size may
>  result in more passes and a 'settling-down period'.  Kept simple for now.
> 
> - Anyone running PPC64 little-endian is doing something both interesting and
>  unsupported for this work :-)  (There are some trivial endian assumptions.)
> 
> Tested in-situ (tcpdump with varying complexity filters) and with a random BPF
> generator; I haven't verified loads from the fall back skb_copy_bits path.  Bug
> reports/testing would be very welcome.

Would be nice to get PPC32 support as well.

- k--
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