linux-kernel - Re: [PATCH v2] powerpc: warn on emulation of dcbz instruction in kernel mode

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Message-ID: <e6acf664-5ebd-4273-9330-cbec283ede23@cs-soprasteria.com>
Date: Thu, 22 Aug 2024 06:39:33 +0000
From: LEROY Christophe <christophe.leroy2@...soprasteria.com>
To: Christoph Hellwig <hch@....de>
CC: Christian Lamparter <christian.lamparter@....uni-stuttgart.de>, Benjamin
 Herrenschmidt <benh@...nel.crashing.org>, Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linuxppc-dev@...ts.ozlabs.org"
	<linuxppc-dev@...ts.ozlabs.org>, Stan Johnson <userm57@...oo.com>, Finn Thain
	<fthain@...ux-m68k.org>
Subject: Re: [PATCH v2] powerpc: warn on emulation of dcbz instruction in
 kernel mode

Le 22/08/2024 à 07:32, Christoph Hellwig a écrit :
> On Thu, Aug 22, 2024 at 05:25:10AM +0000, LEROY Christophe wrote:
>>> and this results in a call to dma_direct_allocation(), which has one
>>> innocent looking memset():
>>
>>
>> memset() can't be used on non-cached memory, memset_io() has to be used
>> instead.
> 
> No, we use memset on uncached memory all the time.  Note that uncached
> memory != __iomem memory, for which you DO have to use memset_io.
> 

Then we have a subject here.

powerpc has a magic instruction 'dcbz' which clears a full cacheline in 
one go. It is far more efficient than a loop to store zeros, and since 
2015 memset(0) has been implemented with that instruction (commit 
5b2a32e80634 ("powerpc/32: memset(0): use cacheable_memzero"))

But that instruction generates an alignment exception when used on 
non-cached memory (whether it is RAM or not doesn't matter). It is then 
emulated by the kernel but it of course leads to a serious performance 
degradation, hence the warning added by commit cbe654c77961 ("powerpc: 
warn on emulation of dcbz instruction in kernel mode"). Until now it 
helped identify and fix use of memset() on IO memory.

But if memset() is expected to be used with non-cached RAM, then I don't 
know what to do. Any suggestion ?

Christophe