linux-kernel - Re: [PATCH] Revert "powerpc/64: Fix checksum folding in csum

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Date:   Thu, 17 May 2018 09:10:52 +1000
From:   Paul Mackerras <paulus@...abs.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Scott Wood <oss@...error.net>, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, Shile Zhang <shile.zhang@...ia.com>
Subject: Re: [PATCH] Revert "powerpc/64: Fix checksum folding in csum_add()"

On Tue, Apr 10, 2018 at 08:34:37AM +0200, Christophe Leroy wrote:
> This reverts commit 6ad966d7303b70165228dba1ee8da1a05c10eefe.
> 
> That commit was pointless, because csum_add() sums two 32 bits
> values, so the sum is 0x1fffffffe at the maximum.
> And then when adding upper part (1) and lower part (0xfffffffe),
> the result is 0xffffffff which doesn't carry.
> Any lower value will not carry either.
> 
> And behind the fact that this commit is useless, it also kills the
> whole purpose of having an arch specific inline csum_add()
> because the resulting code gets even worse than what is obtained
> with the generic implementation of csum_add()
> 
> 0000000000000240 <.csum_add>:
>  240:	38 00 ff ff 	li      r0,-1
>  244:	7c 84 1a 14 	add     r4,r4,r3
>  248:	78 00 00 20 	clrldi  r0,r0,32
>  24c:	78 89 00 22 	rldicl  r9,r4,32,32
>  250:	7c 80 00 38 	and     r0,r4,r0
>  254:	7c 09 02 14 	add     r0,r9,r0
>  258:	78 09 00 22 	rldicl  r9,r0,32,32
>  25c:	7c 00 4a 14 	add     r0,r0,r9
>  260:	78 03 00 20 	clrldi  r3,r0,32
>  264:	4e 80 00 20 	blr
> 
> In comparison, the generic implementation of csum_add() gives:
> 
> 0000000000000290 <.csum_add>:
>  290:	7c 63 22 14 	add     r3,r3,r4
>  294:	7f 83 20 40 	cmplw   cr7,r3,r4
>  298:	7c 10 10 26 	mfocrf  r0,1
>  29c:	54 00 ef fe 	rlwinm  r0,r0,29,31,31
>  2a0:	7c 60 1a 14 	add     r3,r0,r3
>  2a4:	78 63 00 20 	clrldi  r3,r3,32
>  2a8:	4e 80 00 20 	blr
> 
> And the reverted implementation for PPC64 gives:
> 
> 0000000000000240 <.csum_add>:
>  240:	7c 84 1a 14 	add     r4,r4,r3
>  244:	78 80 00 22 	rldicl  r0,r4,32,32
>  248:	7c 80 22 14 	add     r4,r0,r4
>  24c:	78 83 00 20 	clrldi  r3,r4,32
>  250:	4e 80 00 20 	blr
> 
> Fixes: 6ad966d7303b7 ("powerpc/64: Fix checksum folding in csum_add()")
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>

Seems I was right first time... :)

Acked-by: Paul Mackerras <paulus@...abs.org>