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RISC-V: mm: Document mmap changes 

The behavior of mmap is modified with this patch series, so explain the
changes to the mmap hint address behavior.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230809232218.849726-5-charlie@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
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Charlie Jenkins 2023-08-09 16:22:04 -07:00 committed by Palmer Dabbelt
parent 26eee2bfc4
commit 7998abe69d
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Documentation/riscv/vm-layout.rst
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@ -133,3 +133,25 @@ RISC-V Linux Kernel SV57
ffffffff00000000 | -4 GB | ffffffff7fffffff | 2 GB | modules, BPF
ffffffff80000000 | -2 GB | ffffffffffffffff | 2 GB | kernel
__________________|____________|__________________|_________|____________________________________________________________
Userspace VAs
--------------------
To maintain compatibility with software that relies on the VA space with a
maximum of 48 bits the kernel will, by default, return virtual addresses to
userspace from a 48-bit range (sv48). This default behavior is achieved by
passing 0 into the hint address parameter of mmap. On CPUs with an address space
smaller than sv48, the CPU maximum supported address space will be the default.
Software can "opt-in" to receiving VAs from another VA space by providing
a hint address to mmap. A hint address passed to mmap will cause the largest
address space that fits entirely into the hint to be used, unless there is no
space left in the address space. If there is no space available in the requested
address space, an address in the next smallest available address space will be
returned.
For example, in order to obtain 48-bit VA space, a hint address greater than
:code:`1 << 47` must be provided. Note that this is 47 due to sv48 userspace
ending at :code:`1 << 47` and the addresses beyond this are reserved for the
kernel. Similarly, to obtain 57-bit VA space addresses, a hint address greater
than or equal to :code:`1 << 56` must be provided.