[PATCH v8 4/4] RISC-V: mm: Document mmap changes

[Alexandre Ghiti]

On 27/07/2023 23:26, Charlie Jenkins wrote:
> 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 at rivosinc.com>
> ---
>   Documentation/riscv/vm-layout.rst | 22 ++++++++++++++++++++++
>   1 file changed, 22 insertions(+)
>
> diff --git a/Documentation/riscv/vm-layout.rst b/Documentation/riscv/vm-layout.rst
> index 5462c84f4723..69ff6da1dbf8 100644
> --- a/Documentation/riscv/vm-layout.rst
> +++ b/Documentation/riscv/vm-layout.rst
> @@ -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.


You can add:

Reviewed-by: Alexandre Ghiti <alexghiti at rivosinc.com>

Thanks,

Alex