• Yuri Tikhonov's avatar
    powerpc/44x: Support for 256KB PAGE_SIZE · e1240122
    Yuri Tikhonov authored
    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/2008/12/19/20Signed-off-by: default avatarYuri Tikhonov <yur@emcraft.com>
    Signed-off-by: default avatarIlya Yanok <yanok@emcraft.com>
    Signed-off-by: default avatarJosh Boyer <jwboyer@linux.vnet.ibm.com>
    e1240122
page_32.h 1.29 KB