powerpc: Use 64k pages without needing cache-inhibited large pages

Some POWER5+ machines can do 64k hardware pages for normal memory but
not for cache-inhibited pages.  This patch lets us use 64k hardware
pages for most user processes on such machines (assuming the kernel
has been configured with CONFIG_PPC_64K_PAGES=y).  User processes
start out using 64k pages and get switched to 4k pages if they use any
non-cacheable mappings.

With this, we use 64k pages for the vmalloc region and 4k pages for
the imalloc region.  If anything creates a non-cacheable mapping in
the vmalloc region, the vmalloc region will get switched to 4k pages.
I don't know of any driver other than the DRM that would do this,
though, and these machines don't have AGP.

When a region gets switched from 64k pages to 4k pages, we do not have
to clear out all the 64k HPTEs from the hash table immediately.  We
use the _PAGE_COMBO bit in the Linux PTE to indicate whether the page
was hashed in as a 64k page or a set of 4k pages.  If hash_page is
trying to insert a 4k page for a Linux PTE and it sees that it has
already been inserted as a 64k page, it first invalidates the 64k HPTE
before inserting the 4k HPTE.  The hash invalidation routines also use
the _PAGE_COMBO bit, to determine whether to look for a 64k HPTE or a
set of 4k HPTEs to remove.  With those two changes, we can tolerate a
mix of 4k and 64k HPTEs in the hash table, and they will all get
removed when the address space is torn down.

Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 969f4abcc0b..d77d24a89b3 100644
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c
@@ -948,7 +948,10 @@ static struct ibm_pa_feature {
 	{CPU_FTR_CTRL, 0,		0, 3, 0},
 	{CPU_FTR_NOEXECUTE, 0,		0, 6, 0},
 	{CPU_FTR_NODSISRALIGN, 0,	1, 1, 1},
+#if 0
+	/* put this back once we know how to test if firmware does 64k IO */
 	{CPU_FTR_CI_LARGE_PAGE, 0,	1, 2, 0},
+#endif
 };
 
 static void __init check_cpu_pa_features(unsigned long node)