Documentation: riscv: Document the sv57 VM layout

RISC-V has been supporting the "sv57" address translation mode for a
while, but is has not been added to the VM layout documentation. Let
us fix that.
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20221118171556.1612190-1-bjorn@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/riscv/vm-layout.rst

@@ -97,3 +97,39 @@ RISC-V Linux Kernel SV48
    ffffffff00000000 |   -4    GB | ffffffff7fffffff |    2 GB | modules, BPF
    ffffffff80000000 |   -2    GB | ffffffffffffffff |    2 GB | kernel
   __________________|____________|__________________|_________|____________________________________________________________
+
+
+RISC-V Linux Kernel SV57
+------------------------
+
+::
+
+ ========================================================================================================================
+      Start addr    |   Offset   |     End addr     |  Size   | VM area description
+ ========================================================================================================================
+                    |            |                  |         |
+   0000000000000000 |   0        | 00ffffffffffffff |   64 PB | user-space virtual memory, different per mm
+  __________________|____________|__________________|_________|___________________________________________________________
+                    |            |                  |         |
+   0100000000000000 | +64     PB | feffffffffffffff | ~16K PB | ... huge, almost 64 bits wide hole of non-canonical
+                    |            |                  |         | virtual memory addresses up to the -64 PB
+                    |            |                  |         | starting offset of kernel mappings.
+  __________________|____________|__________________|_________|___________________________________________________________
+                                                              |
+                                                              | Kernel-space virtual memory, shared between all processes:
+  ____________________________________________________________|___________________________________________________________
+                    |            |                  |         |
+   ff1bfffffee00000 | -57     PB | ff1bfffffeffffff |    2 MB | fixmap
+   ff1bffffff000000 | -57     PB | ff1bffffffffffff |   16 MB | PCI io
+   ff1c000000000000 | -57     PB | ff1fffffffffffff |    1 PB | vmemmap
+   ff20000000000000 | -56     PB | ff5fffffffffffff |   16 PB | vmalloc/ioremap space
+   ff60000000000000 | -40     PB | ffdeffffffffffff |   32 PB | direct mapping of all physical memory
+   ffdf000000000000 |  -8     PB | fffffffeffffffff |    8 PB | kasan
+  __________________|____________|__________________|_________|____________________________________________________________
+                                                              |
+                                                              | Identical layout to the 39-bit one from here on:
+  ____________________________________________________________|____________________________________________________________
+                    |            |                  |         |
+   ffffffff00000000 |  -4     GB | ffffffff7fffffff |    2 GB | modules, BPF
+   ffffffff80000000 |  -2     GB | ffffffffffffffff |    2 GB | kernel
+  __________________|____________|__________________|_________|____________________________________________________________