[PATCH 2/7] RISC-V: doc: Add file describing the virtual memory map

[Logan Gunthorpe <logang@deltatee.com>]

This file is similar to the x86_64 equivalent (in
Documentation/x86/x86_64/mm.txt) and describes the virtuas address space
usage for RISC-V.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
---
 Documentation/riscv/mm.txt | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)
 create mode 100644 Documentation/riscv/mm.txt

diff --git a/Documentation/riscv/mm.txt b/Documentation/riscv/mm.txt
new file mode 100644
index 000000000000..725dc85f2c65
--- /dev/null
+++ b/Documentation/riscv/mm.txt
@@ -0,0 +1,24 @@
+Sv32:
+
+00000000 - 7fffffff   user space, different per mm (2G)
+80000000 - 81ffffff   virtual memory map (32MB)
+82000000 - bfffffff   vmalloc/ioremap space (1GB - 32MB)
+c0000000 - ffffffff   direct mapping of lower phys. memory (1GB)
+
+Sv39:
+
+0000000000000000 - 0000003fffffffff  user space, different per mm (256GB)
+hole caused by [38:63] sign extension
+ffffffc000000000 - ffffffc0ffffffff  virtual memory map (4GB)
+ffffffc100000000 - ffffffd0ffffffff  vmalloc/ioremap spac (64GB)
+ffffffd100000000 - ffffffffffffffff  linear mapping of physical space (188GB)
+  ffffffd200000000 - 0xfffffff200000000 linear mapping of all physical memory
+
+The RISC-V architecture defines virtual address bits in multiples of nine
+starting from 39. These are referred to as Sv39, Sv48, Sv57 and Sv64.
+Currently only Sv39 is supported. Bits 63 through to the most-significant
+implemented bit are sign extended. This causes a hole between user space
+and kernel addresses if you interpret them as unsigned.
+
+The direct mapping covers as much of the physical memory space as
+possible so that it may cover some IO memory.
-- 
2.20.1