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

On Thu, 2015-05-14 at 10:50 +0200, christophe leroy wrote:

Le 14/05/2015 02:55, Scott Wood a écrit :

quoted

On Tue, 2015-05-12 at 15:32 +0200, Christophe Leroy wrote:

quoted

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 <redacted>
---
  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 ?

If you want to do it based on the virtual address, rather than doing a
tablewalk or TLB search, you need to limit it to lowmem.

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
}

Right, the problem is figuring out the pfn in the first place.

quoted

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 ?

Maybe.  If that's not a safe assumption I hope someone will point it
out.

copy_tofrom_user() already does this assumption (allthought a user app 
could possibly provide a buffer located in an ALSA mapped IO area)

The user could also pass in NULL.  That's what the fixups are for. :-)

-Scott