netdev - Re: [PATCH] net: filter: Convert the BPF VM to threaded code

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Message-ID: <20110801181652.GB2732@nuttenaction>
Date:	Mon, 1 Aug 2011 20:16:52 +0200
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	Rui Ueyama <rui314@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] net: filter: Convert the BPF VM to threaded code

* Rui Ueyama | 2011-07-29 01:10:26 [-0700]:

>Convert the BPF VM to threaded code to improve performance.
>
>The BPF VM is basically a big for loop containing a switch statement.  That is
>slow because for each instruction it checks the for loop condition and does the
>conditional branch of the switch statement.
>
>This patch eliminates the conditional branch, by replacing it with jump table
>using GCC's labels-as-values feature. The for loop condition check can also be
>removed, because the filter code always end with a RET instruction.

With commit 01f2f3f6ef4d076c I reworked the BPF code so that gcc is in the
ability to generate a jump table, I double checked this. Not sure what happened
in the meantime.

Hagen


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