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

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Message-ID: <c8127b03-245b-4eaa-98f5-6e14df094669@cs-soprasteria.com>
Date: Sat, 24 Aug 2024 09:01:33 +0000
From: LEROY Christophe <christophe.leroy2@...soprasteria.com>
To: Segher Boessenkool <segher@...nel.crashing.org>, 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 23/08/2024 à 21:19, Segher Boessenkool a écrit :
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> 
> Hi!
> 
> On Fri, Aug 23, 2024 at 03:54:59PM +0200, Christoph Hellwig wrote:
>> On Fri, Aug 23, 2024 at 08:06:00AM -0500, Segher Boessenkool wrote:
>>> What does "uncached memory" even mean here?  Literally it would be
>>> I=1 memory (uncachEABLE memory), but more likely you want M=0 memory
>>> here ("non-memory memory", "not well-behaved memory", MMIO often).
>>
>> Regular kernel memory vmapped with pgprot_noncached().
> 
> So, I=1 (and G=1).  Caching inhibited and guarded.  But M=1 (memory
> coherence required) as with any other real memory :-)
> 
>>> If memset() is expected to be used with M=0, you cannot do any serious
>>> optimisations to it at all.  If memset() is expected to be used with I=1
>>> it should use a separate code path for it, probably the caller should
>>> make the distinction.
>>
>> DMA coherent memory which uses uncached memory for platforms that
>> do not provide hardware dma coherence can end up just about anywhere
>> in the kernel.  We could use special routines for a few places in
>> the DMA subsystem, but there might be plenty of others.
> 
> Yeah.  It will just be plenty slow, as we see here, that's what the
> warning is for; but it works just fine :-)
> 
> The memset() code itself could chech for the storage attributes, but
> that is probably more expensive than just assuming the happy case.
> Maybe someone could try it out though!

But is it only memset() the problem ?

dcbz instruction is also used in:
- memcpy()
- csum_partial_copy_generic()
- clear_page()
- copy_page()
- clear_user()
- copy_to_user()
- copy_from_user()

Are these functions also used on DMA coherent memory ?

Christophe