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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@ -144,14 +144,8 @@ passing 0 into the hint address parameter of mmap. On CPUs with an address space
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smaller than sv48, the CPU maximum supported address space will be the default.
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Software can "opt-in" to receiving VAs from another VA space by providing
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a hint address to mmap. A hint address passed to mmap will cause the largest
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address space that fits entirely into the hint to be used, unless there is no
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space left in the address space. If there is no space available in the requested
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address space, an address in the next smallest available address space will be
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returned.
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For example, in order to obtain 48-bit VA space, a hint address greater than
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:code:`1 << 47` must be provided. Note that this is 47 due to sv48 userspace
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ending at :code:`1 << 47` and the addresses beyond this are reserved for the
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kernel. Similarly, to obtain 57-bit VA space addresses, a hint address greater
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than or equal to :code:`1 << 56` must be provided.
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a hint address to mmap. When a hint address is passed to mmap, the returned
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address will never use more bits than the hint address. For example, if a hint
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address of `1 << 40` is passed to mmap, a valid returned address will never use
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bits 41 through 63. If no mappable addresses are available in that range, mmap
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will return `MAP_FAILED`.
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