netdev - Re: [PATCH 2/3] net: bpf jit: x86: optimize choose_load

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Message-ID: <20131014025535.GA1870@hp530>
Date:	Mon, 14 Oct 2013 04:55:39 +0200
From:	Vladimir Murzin <murzin.v@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	av1474@...tv.ru
Subject: Re: [PATCH 2/3] net: bpf jit: x86: optimize choose_load_func error
 path

On Sun, Oct 13, 2013 at 09:36:34AM -0700, Eric Dumazet wrote:
> On Sun, 2013-10-13 at 16:54 +0200, Vladimir Murzin wrote:
> > Macro CHOOSE_LOAD_FUNC returns handler for "any offset" if checks for K
> > were not passed. At the same time handlers for "any offset" cases make
> > the same checks against r_addr at run-time, that will always lead to
> > bpf_error.
> > 
> > Run-time checks are still necessary for indirect load operations, but
> > error path for absolute and mesh loads are worth to optimize during bpf
> > compile time.
> 
> I don't get the point.
> 
> What real world use case or problem are you trying to handle ?
> 
> bpf_error returns 0, so it seems your patch does the same.
> 
> A buggy BPF program should not expect us to 'save' a few cycles.
> 
> 
> 

Hi Eric!

There is no real world use case for me - it was eliminated by plain code
reading. The patch is not supposed to change behavior of BPF program - only
optimization of the error path. 
I agree with, you there is no significant reason for optimizations of rarely
used pice of code. However, it is not only saving pipeline cycles and I-cache
lines for usually "never taken" branch. In case this "never taken" branch is a
buggy part of BPF program, we can avoid extra instructions and save space for
the rest of BPF program - there is no need to care about seen flags of buggy
part anymore.

Anyway, if you still think it's not good enough - just throw it away ;)

Thanks
Vladimir
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