MIPS: Add 48-bit VA space (and 4-level page tables) for 4K pages.

Some users must have 4K pages while needing a 48-bit VA space size.
The cleanest way do do this is to go to a 4-level page table for this
case.  Each page table level using order-0 pages adds 9 bits to the
VA size (at 4K pages, so for four levels we get 9 * 4 + 12 == 48-bits.

For the 4K page size case only we add support functions for the PUD
level of the page table tree, also the TLB exception handlers get an
extra level of tree walk.

[david.daney@cavium.com: Forward port to v4.10.]
[david.daney@cavium.com: Forward port to v4.11.]

Signed-off-by: Alex Belits <alex.belits@cavium.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Alex Belits <alex.belits@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15312/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 8 insertions, 5 deletions

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 68af16b72e9c..f4dd2c322d4b 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -2121,10 +2121,13 @@ config MIPS_VA_BITS_48
         bool "48 bits virtual memory"
         depends on 64BIT
         help
-          Support a maximum at least 48 bits of application virtual memory.
-          Default is 40 bits or less, depending on the CPU.
-          This option result in a small memory overhead for page tables.
-          This option is only supported with 16k and 64k page sizes.
+          Support a maximum at least 48 bits of application virtual
+          memory.  Default is 40 bits or less, depending on the CPU.
+          For page sizes 16k and above, this option results in a small
+          memory overhead for page tables.  For 4k page size, a fourth
+          level of page tables is added which imposes both a memory
+          overhead as well as slower TLB fault handling.
+
           If unsure, say N.
 
 choice
@@ -2134,7 +2137,6 @@ choice
 config PAGE_SIZE_4KB
         bool "4kB"
         depends on !CPU_LOONGSON2 && !CPU_LOONGSON3
-        depends on !MIPS_VA_BITS_48
         help
          This option select the standard 4kB Linux page size.  On some
          R3000-family processors this is the only available page size.  Using
@@ -2983,6 +2985,7 @@ config HAVE_LATENCYTOP_SUPPORT
 
 config PGTABLE_LEVELS
         int
+        default 4 if PAGE_SIZE_4KB && MIPS_VA_BITS_48
         default 3 if 64BIT && !PAGE_SIZE_64KB
         default 2