netdev - Re: [PATCH net] bpf: do not use reciprocal divide

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Date:	Tue, 14 Jan 2014 23:39:36 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	hannes@...essinduktion.org, netdev@...r.kernel.org,
	dborkman@...hat.com, darkjames-ws@...kjames.pl, mgherzan@...il.com,
	rmk+kernel@....linux.org.uk, matt@...abs.org,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com
Subject: Re: [PATCH net] bpf: do not use reciprocal divide

On Tue, 2014-01-14 at 23:28 -0800, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Tue, 14 Jan 2014 23:02:41 -0800
> 
> > diff --git a/arch/sparc/net/bpf_jit_comp.c b/arch/sparc/net/bpf_jit_comp.c
> > index 218b6b23c378..125045063b91 100644
> > --- a/arch/sparc/net/bpf_jit_comp.c
> > +++ b/arch/sparc/net/bpf_jit_comp.c
> > @@ -498,8 +498,7 @@ void bpf_jit_compile(struct sk_filter *fp)
> >  				emit_alu_K(MUL, K);
> >  				break;
> >  			case BPF_S_ALU_DIV_K:	/* A /= K */
> > -				emit_alu_K(MUL, K);
> > -				emit_read_y(r_A);
> > +				emit_alu_K(DIV, K);
> >  				break;
> >  			case BPF_S_ALU_DIV_X:	/* A /= X; */
> >  				emit_cmpi(r_X, 0);
> 
> You have to clear the Y register before a divide, as it provides
> the top 32-bits of the 64-bit numerator.
> 
> You can just cut and paste the sequence used for BPF_S_ALU_DIV_X:
> 
> 				emit_write_y(G0);
> #ifdef CONFIG_SPARC32
> 				/* The Sparc v8 architecture requires
> 				 * three instructions between a %y
> 				 * register write and the first use.
> 				 */
> 				emit_nop();
> 				emit_nop();
> 				emit_nop();
> #endif
> 				emit_alu_X(DIV);

Thanks David, I will submit a v2 tomorrow morning, when/if I get more
feedback for other arches, and after some rest ;)



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