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

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Date:	Tue, 05 Jan 2016 14:18:33 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <tom@...bertland.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, kernel-team@...com,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com, x86@...nel.org
Subject: Re: [PATCH v2 net-next] net: Implement fast csum_partial for x86_64

On Tue, 2016-01-05 at 10:41 -0800, Tom Herbert wrote:
> Implement assembly routine for csum_partial for 64 bit x86. This
> primarily speeds up checksum calculation for smaller lengths such as
> those that are present when doing skb_postpull_rcsum when getting
> CHECKSUM_COMPLETE from device or after CHECKSUM_UNNECESSARY
> conversion.

Very nice !


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 */


Acked-by: Eric Dumazet <edumazet@...gle.com>


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