Re: [PATCH] Documentation/arm64: add memory layout with 4KB pages + VA39-bit

On Wed, 6 Oct 2021 18:14:38 +0100
Catalin Marinas [off-list ref] wrote:

On Thu, Sep 30, 2021 at 06:50:26PM +0800, Jisheng Zhang wrote:

quoted

The 4KB pages + 3 levels (39-bit) combination is also widely used in
arm64 world, add the memory layout description for this combination.

Signed-off-by: Jisheng Zhang <redacted>
---
 Documentation/arm64/memory.rst | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Documentation/arm64/memory.rst b/Documentation/arm64/memory.rst
index 901cd094f4ec..d1745b570f0c 100644
--- a/Documentation/arm64/memory.rst
+++ b/Documentation/arm64/memory.rst

@@ -26,6 +26,23 @@ The swapper_pg_dir address is written to TTBR1 and never written to
 TTBR0.


+AArch64 Linux memory layout with 4KB pages + 3 levels (39-bit)::
+  Start                      End                     Size            Use
+  -----------------------------------------------------------------------
+  0000000000000000   0000007fffffffff         512GB          user
+  ffffff8000000000   ffffffbfffffffff         256GB          kernel logical memory map
+ [ffffffb000000000   ffffffbfffffffff]         64GB          [kasan shadow region]
+  ffffffc000000000   ffffffc007ffffff         128MB          bpf jit region
+  ffffffc008000000   ffffffc00fffffff         128MB          modules
+  ffffffc010000000   fffffffdefffffff      253440MB          vmalloc
+  fffffffdf0000000   fffffffdfdffffff         224MB          fixed mappings (top down)
+  fffffffdfe000000   fffffffdfe7fffff           8MB          [guard region]
+  fffffffdfe800000   fffffffdff7fffff          16MB          PCI I/O space
+  fffffffdff800000   fffffffdffffffff           8MB          [guard region]
+  fffffffe00000000   ffffffffefffffff           4GB          vmemmap
+  ffffffff00000000   ffffffffffffffff           4GB          [guard region]  

I wouldn't bother maintaining these. There are other combinations that
people may use. The 4KB + 48-bit VA is defconfig while 64KB + 52-bit was
more interesting, so we thought it would be useful.

If kernel config file isn't based on the defconfig, the 4KB + 39bit VA is the
default option in Kconfig:

...
        prompt "Page size"
        default ARM64_4K_PAGES
        help
          Page size (translation granule) configuration.

...

choice
        prompt "Virtual address space size"
        default ARM64_VA_BITS_39 if ARM64_4K_PAGES
        default ARM64_VA_BITS_47 if ARM64_16K_PAGES
        default ARM64_VA_BITS_42 if ARM64_64K_PAGES
        help
          Allows choosing one of multiple possible virtual address


I believe this combination is widely used if 48bit VA isn't necessary. In
fact, I think the default android GKI image is built with 4KB + 39bit VA. This
is the reason why I added description for 4K + VA39 but leave other
combinations.

I'm more inclined to remove them altogether and maybe just add some
high-level ascii art as per the log of commit f4693c2716b3 ("arm64: mm:
extend linear region for 52-bit VA configurations").

The detailed memory layout is useful, for example, I can immediately know
which region the virtual address belongs to when checking kernel panic logs.
If changed to high level ascii, I need to do some calculation, sometimes I
need to check source code to get the value of some MACROS defitions.