arm64: mm: extend linear region for 52-bit VA configurations

For historical reasons, the arm64 kernel VA space is configured as two
equally sized halves, i.e., on a 48-bit VA build, the VA space is split
into a 47-bit vmalloc region and a 47-bit linear region.

When support for 52-bit virtual addressing was added, this equal split
was kept, resulting in a substantial waste of virtual address space in
the linear region:

                           48-bit VA                     52-bit VA
  0xffff_ffff_ffff_ffff +-------------+               +-------------+
                        |   vmalloc   |               |   vmalloc   |
  0xffff_8000_0000_0000 +-------------+ _PAGE_END(48) +-------------+
                        |   linear    |               :             :
  0xffff_0000_0000_0000 +-------------+               :             :
                        :             :               :             :
                        :             :               :             :
                        :             :               :             :
                        :             :               :  currently  :
                        :  unusable   :               :             :
                        :             :               :   unused    :
                        :     by      :               :             :
                        :             :               :             :
                        :  hardware   :               :             :
                        :             :               :             :
  0xfff8_0000_0000_0000 :             : _PAGE_END(52) +-------------+
                        :             :               |             |
                        :             :               |             |
                        :             :               |             |
                        :             :               |             |
                        :             :               |             |
                        :  unusable   :               |             |
                        :             :               |   linear    |
                        :     by      :               |             |
                        :             :               |   region    |
                        :  hardware   :               |             |
                        :             :               |             |
                        :             :               |             |
                        :             :               |             |
                        :             :               |             |
                        :             :               |             |
                        :             :               |             |
  0xfff0_0000_0000_0000 +-------------+  PAGE_OFFSET  +-------------+

As illustrated above, the 52-bit VA kernel uses 47 bits for the vmalloc
space (as before), to ensure that a single 64k granule kernel image can
support any 64k granule capable system, regardless of whether it supports
the 52-bit virtual addressing extension. However, due to the fact that
the VA space is still split in equal halves, the linear region is only
2^51 bytes in size, wasting almost half of the 52-bit VA space.

Let's fix this, by abandoning the equal split, and simply assigning all
VA space outside of the vmalloc region to the linear region.

The KASAN shadow region is reconfigured so that it ends at the start of
the vmalloc region, and grows downwards. That way, the arrangement of
the vmalloc space (which contains kernel mappings, modules, BPF region,
the vmemmap array etc) is identical between non-KASAN and KASAN builds,
which aids debugging.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Link: https://lore.kernel.org/r/20201008153602.9467-3-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 10 insertions, 10 deletions

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 1515f6f153a0..c6092cbb39af 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -331,16 +331,16 @@ config BROKEN_GAS_INST
 config KASAN_SHADOW_OFFSET
 	hex
 	depends on KASAN
-	default 0xdfffa00000000000 if (ARM64_VA_BITS_48 || ARM64_VA_BITS_52) && !KASAN_SW_TAGS
-	default 0xdfffd00000000000 if ARM64_VA_BITS_47 && !KASAN_SW_TAGS
-	default 0xdffffe8000000000 if ARM64_VA_BITS_42 && !KASAN_SW_TAGS
-	default 0xdfffffd000000000 if ARM64_VA_BITS_39 && !KASAN_SW_TAGS
-	default 0xdffffffa00000000 if ARM64_VA_BITS_36 && !KASAN_SW_TAGS
-	default 0xefff900000000000 if (ARM64_VA_BITS_48 || ARM64_VA_BITS_52) && KASAN_SW_TAGS
-	default 0xefffc80000000000 if ARM64_VA_BITS_47 && KASAN_SW_TAGS
-	default 0xeffffe4000000000 if ARM64_VA_BITS_42 && KASAN_SW_TAGS
-	default 0xefffffc800000000 if ARM64_VA_BITS_39 && KASAN_SW_TAGS
-	default 0xeffffff900000000 if ARM64_VA_BITS_36 && KASAN_SW_TAGS
+	default 0xdfff800000000000 if (ARM64_VA_BITS_48 || ARM64_VA_BITS_52) && !KASAN_SW_TAGS
+	default 0xdfffc00000000000 if ARM64_VA_BITS_47 && !KASAN_SW_TAGS
+	default 0xdffffe0000000000 if ARM64_VA_BITS_42 && !KASAN_SW_TAGS
+	default 0xdfffffc000000000 if ARM64_VA_BITS_39 && !KASAN_SW_TAGS
+	default 0xdffffff800000000 if ARM64_VA_BITS_36 && !KASAN_SW_TAGS
+	default 0xefff800000000000 if (ARM64_VA_BITS_48 || ARM64_VA_BITS_52) && KASAN_SW_TAGS
+	default 0xefffc00000000000 if ARM64_VA_BITS_47 && KASAN_SW_TAGS
+	default 0xeffffe0000000000 if ARM64_VA_BITS_42 && KASAN_SW_TAGS
+	default 0xefffffc000000000 if ARM64_VA_BITS_39 && KASAN_SW_TAGS
+	default 0xeffffff800000000 if ARM64_VA_BITS_36 && KASAN_SW_TAGS
 	default 0xffffffffffffffff
 
 source "arch/arm64/Kconfig.platforms"