netdev - Re: [PATCH net-next-2.6 v2] filter: optimize sk_run

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Date:	Fri, 19 Nov 2010 15:35:06 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	xiaosuo@...il.com, davem@...emloft.net, hagen@...u.net,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next-2.6 v2] filter: optimize sk_run_filter

Le vendredi 19 novembre 2010 à 23:17 +0900, Tetsuo Handa a écrit :
> Just my thought again...
> 
> Eric Dumazet wrote:
> > @@ -167,34 +168,36 @@ unsigned int sk_run_filter(struct sk_buff *skb, struct sock_filter *filter, int
> >  	unsigned long memvalid = 0;
> >  	u32 tmp;
> >  	int k;
> 
> Is this 'k' useful?
> 

Yes this is useful, please read later we have :

	k = X + K;
	goto load_w;

K is readonly, k is a rw temp variable (on stack)

> > -	int pc;
> >  
> >  	BUILD_BUG_ON(BPF_MEMWORDS > BITS_PER_LONG);
> >  	/*
> >  	 * Process array of filter instructions.
> >  	 */
> > -	for (pc = 0; pc < flen; pc++) {
> > -		const struct sock_filter *fentry = &filter[pc];
> > -		u32 f_k = fentry->k;
> > +	for (;; fentry++) {
> > +#if defined(CONFIG_X86_32)
> > +#define	K (fentry->k)
> > +#else
> > +		const u32 K = fentry->k;
> 
> What happens if we use
> 
> 	u32 f_k = fentry->k;
> 
> and
> 
> >  		case BPF_S_LD_W_ABS:
> > -			k = f_k;
> > +			k = K;
> 
> remove this assignment and
> 
> >  load_w:
> >  			ptr = load_pointer(skb, k, 4, &tmp);
> 
> change to
> 
> 	ptr = load_pointer(skb, (int) f_k, 4, &tmp);
> 
> and
> 
> >  		case BPF_S_LD_W_IND:
> > -			k = X + f_k;
> > +			k = X + K;
> 
> change to
> 
> 	f_k += X;
> 
> ?
> >  			goto load_w;


This wont work, K is really a constant, part of the filter program,
while k is the (existing) temp variable.


 


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