netdev - Re: [PATCH v2 net-next] net: Implement fast csum_partial for x86

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Message-ID: <1452090307.8255.173.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 06 Jan 2016 06:25:07 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Tom Herbert <tom@...bertland.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"kernel-team@...com" <kernel-team@...com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v2 net-next] net: Implement fast csum_partial for x86_64

On Wed, 2016-01-06 at 10:16 +0000, David Laight wrote:
> From: Eric Dumazet
> > Sent: 05 January 2016 22:19
> > To: Tom Herbert
> > You might add a comment telling the '4' comes from length of 'adcq
> > 6*8(%rdi),%rax' instruction, and that the 'nop' is to compensate that
> > 'adcq    0*8(%rdi),%rax' is using 3 bytes instead.
> > 
> > We also could use .byte 0x48, 0x13, 0x47, 0x00 to force a 4 bytes
> > instruction and remove the nop.
> > 
> > 
> > +       lea     20f(, %rcx, 4), %r11
> > +       clc
> > +       jmp     *%r11
> > +
> > +.align 8
> > +       adcq    6*8(%rdi),%rax
> > +       adcq    5*8(%rdi),%rax
> > +       adcq    4*8(%rdi),%rax
> > +       adcq    3*8(%rdi),%rax
> > +       adcq    2*8(%rdi),%rax
> > +       adcq    1*8(%rdi),%rax
> > +       adcq    0*8(%rdi),%rax // could force a 4 byte instruction (.byte 0x48, 0x13, 0x47, 0x00)
> > +       nop
> > +20:    /* #quads % 8 jump table base */
> 
> Or move label at the top (after the .align) and adjust the maths.
> You could add a second label after the first adcq and use the
> difference between them for the '4'.

Not really.

We could not use the trick it the length was 5.

Only 1, 2, 4 and 8 are supported.


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