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docs: riscv: Define behavior of mmap 

Define mmap on riscv to not provide an address that uses more bits than
the hint address, if provided.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20240130-use_mmap_hint_address-v3-3-8a655cfa8bcb@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
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Charlie Jenkins 2024-01-30 17:07:02 -08:00 committed by Palmer Dabbelt
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commit 371a3c2055
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Documentation/arch/riscv/vm-layout.rst
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@ -144,14 +144,8 @@ 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.
a hint address to mmap. When a hint address is passed to mmap, the returned
address will never use more bits than the hint address. For example, if a hint
address of `1 << 40` is passed to mmap, a valid returned address will never use
bits 41 through 63. If no mappable addresses are available in that range, mmap
will return `MAP_FAILED`.