linux-kernel - Re: [PATCH 3/4] powerpc32: memset(0): use cacheable

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Date:	Thu, 14 May 2015 10:50:07 +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@...fetthome.net>
Subject: Re: [PATCH 3/4] powerpc32: memset(0): use cacheable_memzero



Le 14/05/2015 02:55, Scott Wood a écrit :
> On Tue, 2015-05-12 at 15:32 +0200, Christophe Leroy wrote:
>> cacheable_memzero uses dcbz instruction and is more efficient than
>> memset(0) when the destination is in RAM
>>
>> This patch renames memset as generic_memset, and defines memset
>> as a prolog to cacheable_memzero. This prolog checks if the byte
>> to set is 0 and if the buffer is in RAM. If not, it falls back to
>> generic_memcpy()
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>> ---
>>   arch/powerpc/lib/copy_32.S | 15 ++++++++++++++-
>>   1 file changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/lib/copy_32.S b/arch/powerpc/lib/copy_32.S
>> index cbca76c..d8a9a86 100644
>> --- a/arch/powerpc/lib/copy_32.S
>> +++ b/arch/powerpc/lib/copy_32.S
>> @@ -12,6 +12,7 @@
>>   #include <asm/cache.h>
>>   #include <asm/errno.h>
>>   #include <asm/ppc_asm.h>
>> +#include <asm/page.h>
>>   
>>   #define COPY_16_BYTES		\
>>   	lwz	r7,4(r4);	\
>> @@ -74,6 +75,18 @@ CACHELINE_MASK = (L1_CACHE_BYTES-1)
>>    * to set them to zero.  This requires that the destination
>>    * area is cacheable.  -- paulus
>>    */
>> +_GLOBAL(memset)
>> +	cmplwi	r4,0
>> +	bne-	generic_memset
>> +	cmplwi	r5,L1_CACHE_BYTES
>> +	blt-	generic_memset
>> +	lis	r8,max_pfn@ha
>> +	lwz	r8,max_pfn@l(r8)
>> +	tophys	(r9,r3)
>> +	srwi	r9,r9,PAGE_SHIFT
>> +	cmplw	r9,r8
>> +	bge-	generic_memset
>> +	mr	r4,r5
> max_pfn includes highmem, and tophys only works on normal kernel
> addresses.
Is there any other simple way to determine whether an address is in RAM 
or not ?

I did that because of the below function from mm/mem.c

|int  page_is_ram(unsigned long  pfn)
{
#ifndef CONFIG_PPC64	/* XXX for now */
	return  pfn<  max_pfn;
#else
	unsigned long  paddr= (pfn<<  PAGE_SHIFT);
	struct  memblock_region*reg;

	for_each_memblock(memory,  reg)
		if  (paddr>=  reg->base&&  paddr< (reg->base+  reg->size))
			return  1;
	return  0;
#endif
}
|



>
> If we were to point memset_io, memcpy_toio, etc. at noncacheable
> versions, are there any other callers left that can reasonably point at
> uncacheable memory?
Do you mean we could just consider that memcpy() and memset() are called 
only with destination on RAM and thus we could avoid the check ?
copy_tofrom_user() already does this assumption (allthought a user app 
could possibly provide a buffer located in an ALSA mapped IO area)

Christophe


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