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

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Date:	Sun, 13 Oct 2013 09:36:34 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Vladimir Murzin <murzin.v@...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, 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.



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