netdev - Re: [PATCH v2] net: filter: Just In Time compiler

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Date:	Thu, 14 Apr 2011 18:41:47 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Hagen Paul Pfeifer <hagen@...u.net>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Ben Hutchings <bhutchings@...arflare.com>
Subject: Re: [PATCH v2] net: filter: Just In Time compiler

On 04/14/2011 05:55 PM, Hagen Paul Pfeifer wrote:
> On Thu, 14 Apr 2011 17:40:03 +0300, Avi Kivity<avi@...hat.com>  wrote:
>
> >  Have you considered putting the compiler in userspace?
>
> Kernelspace (modules, threads, etc) can register BPF filters too. It is
> possible that there is no userspace involved at all.

A userspace jit would still work just fine, no?  I don't want the user 
who supplied the program to also supply the jit; rather, when the kernel 
installs the bpf program, it also asks an independent userspace compiler 
to translate it.

> >  The upside is that you can use established optimizing compilers like
> >  LLVM or GCC, which already support more target architectures.  It may
> >  not matter much for something simple like bpf, but other VMs may be a
> >  lot more complicated.
>
> BPF is another domain. Standard compiler optimization are not comparable
> to BPF optimizations so there is no gain there. Maybe writing a gcc front
> _and_ back-end may gain some valuable advantages.

I'm talking about optimizing the generated code.  For example, bpf has 
just two registers so a complex program generates a lot of loads and 
stores.  An optimizing compiler can use extra target registers to avoid 
those spills, and doesn't need to keep A and X in fixed registers.

If you translate the bpf program to C and optimize that with gcc you'll 
probably get much better machine code that the jit in the patch.

-- 
error compiling committee.c: too many arguments to function

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