linux-kernel - Re: [PATCH 1/4] Partially revert "powerpc: Remove duplicate cacheable

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Date:	Fri, 15 May 2015 19:58:27 +0200
From:	christophe leroy <christophe.leroy@....fr>
To:	Scott Wood <scottwood@...escale.com>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>,
	Kyle Moffett <Kyle.D.Moffett@...ing.com>
Subject: Re: [PATCH 1/4] Partially revert "powerpc: Remove duplicate cacheable_memcpy/memzero
 functions"


Le 14/05/2015 02:49, Scott Wood a écrit :
> On Tue, 2015-05-12 at 15:32 +0200, Christophe Leroy wrote:
>> This partially reverts
>> commit 'powerpc: Remove duplicate cacheable_memcpy/memzero functions
>> ("f909a35bdfb7cb350d078a2cf888162eeb20381c")'
> I don't have that SHA.  Do you mean
> b05ae4ee602b7dc90771408ccf0972e1b3801a35?
Right, took it from the wrong tree sorry.
>
>> Functions cacheable_memcpy/memzero are more efficient than
>> memcpy/memset as they use the dcbz instruction which avoids refill
>> of the cacheline with the data that we will overwrite.
> I don't see anything in this patchset that addresses the "NOTE: The old
> routines are just flat buggy on kernels that support hardware with
> different cacheline sizes" comment.
I believe the NOTE means that if a kernel is compiled for several CPUs 
having different cache line size,
then it will not work. But it is also the case of other functions using 
dcbz instruction, like copy_page() clear_page() copy_tofrom_user().

And indeed, this seems only possible in three cases:
1/ With CONFIG_44x as 47x has different size than 44x and 46x. However 
it is explicitly stated in arch/powerpc/platforms/44x/Kconfig : "config 
PPC_47x This option enables support for the 47x family of processors and 
is not currently compatible with other 44x or 46x varients"
2/ With CONFIG_PPC_85xx, as PPC_E500MC has different size than other 
E500. However it is explicitly stated in 
arch/powerpc/platforms/Kconfig.cputype : "config PPC_E500MC This must be 
enabled for running on e500mc (and derivatives such as e5500/e6500), and 
must be disabled for running on e500v1 or e500v2."
3/ With CONFIG_403GCX as 403GCX has different size than other 40x. 
However it seems to be no way to select CONFIG_403GCX from 
arch/powerpc/platforms/40x/Kconfig

Christophe

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