powerpc/44x: Support for 256KB PAGE_SIZE

This patch adds support for 256KB pages on ppc44x-based boards.

For simplification of implementation with 256KB pages we still assume
2-level paging. As a side effect this leads to wasting extra memory space
reserved for PTE tables: only 1/4 of pages allocated for PTEs are
actually used. But this may be an acceptable trade-off to achieve the
high performance we have with big PAGE_SIZEs in some applications (e.g.
RAID).

Also with 256KB PAGE_SIZE we increase THREAD_SIZE up to 32KB to minimize
the risk of stack overflows in the cases of on-stack arrays, which size
depends on the page size (e.g. multipage BIOs, NTFS, etc.).

With 256KB PAGE_SIZE we need to decrease the PKMAP_ORDER at least down
to 9, otherwise all high memory (2 ^ 10 * PAGE_SIZE == 256MB) we'll be
occupied by PKMAP addresses leaving no place for vmalloc. We do not
separate PKMAP_ORDER for 256K from 16K/64K PAGE_SIZE here; actually that
value of 10 in support for 16K/64K had been selected rather intuitively.
Thus now for all cases of PAGE_SIZE on ppc44x (including the default, 4KB,
one) we have 512 pages for PKMAP.

Because ELF standard supports only page sizes up to 64K, then you should
use binutils later than 2.17.50.0.3 with '-zmax-page-size' set to 256K
for building applications, which are to be run with the 256KB-page sized
kernel. If using the older binutils, then you should patch them like follows:

	--- binutils/bfd/elf32-ppc.c.orig
	+++ binutils/bfd/elf32-ppc.c

	-#define ELF_MAXPAGESIZE                0x10000
	+#define ELF_MAXPAGESIZE                0x40000

One more restriction we currently have with 256KB page sizes is inability
to use shmem safely, so, for now, the 256KB is available only if you turn
the CONFIG_SHMEM option off (another variant is to use BROKEN).
Though, if you need shmem with 256KB pages, you can always remove the !SHMEM
dependency in 'config PPC_256K_PAGES', and use the workaround available here:
 http://lkml.org/lkml/2008/12/19/20

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>
Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index ccdd8de..2d6d133 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -409,6 +409,18 @@ config PPC_HAS_HASH_64K
 	depends on PPC64
 	default n
 
+config STDBINUTILS
+	bool "Using standard binutils settings"
+	depends on 44x
+	default y
+	help
+	  Turning this option off allows you to select 256KB PAGE_SIZE on 44x.
+	  Note, that kernel will be able to run only those applications,
+	  which had been compiled using binutils later than 2.17.50.0.3 with
+	  '-zmax-page-size' set to 256K (the default is 64K). Or, if using
+	  the older binutils, you can patch them with a trivial patch, which
+	  changes the ELF_MAXPAGESIZE definition from 0x10000 to 0x40000.
+
 choice
 	prompt "Page size"
 	default PPC_4K_PAGES
@@ -444,6 +456,19 @@ config PPC_64K_PAGES
 	bool "64k page size" if 44x || PPC_STD_MMU_64
 	select PPC_HAS_HASH_64K if PPC_STD_MMU_64
 
+config PPC_256K_PAGES
+	bool "256k page size" if 44x
+	depends on !STDBINUTILS && (!SHMEM || BROKEN)
+	help
+	  Make the page size 256k.
+
+	  As the ELF standard only requires alignment to support page
+	  sizes up to 64k, you will need to compile all of your user
+	  space applications with a non-standard binutils settings
+	  (see the STDBINUTILS description for details).
+
+	  Say N unless you know what you are doing.
+
 endchoice
 
 config FORCE_MAX_ZONEORDER
@@ -456,6 +481,8 @@ config FORCE_MAX_ZONEORDER
 	default "9" if PPC_STD_MMU_32 && PPC_16K_PAGES
 	range 7 64 if PPC_STD_MMU_32 && PPC_64K_PAGES
 	default "7" if PPC_STD_MMU_32 && PPC_64K_PAGES
+	range 5 64 if PPC_STD_MMU_32 && PPC_256K_PAGES
+	default "5" if PPC_STD_MMU_32 && PPC_256K_PAGES
 	range 11 64
 	default "11"
 	help