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

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Date:	Wed, 13 May 2015 19:55:09 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Christophe Leroy <christophe.leroy@....fr>
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.D.Moffett@...ing.com>
Subject: Re: [PATCH 3/4] powerpc32: memset(0): use cacheable_memzero

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.

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?

-Scott


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