Re: [PATCH 3/4] powerpc32: memset(0): use cacheable_memzero

[christophe leroy]

Le 14/05/2015 02:55, Scott Wood a Ãcrit :
> 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@xxxxxx>
> > ---
> >   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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