[PATCH v4 4/4] RISC-V: mm: Document mmap changes

[Randy Dunlap]

Hi--

On 7/7/23 18:11, 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..8141addbf888 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 other VA space by providing

                                          from another VA space
or
                                          from other VA spaces
I prefer the first one.

> +a hint address to mmap. A call to mmap is guaranteed to return an address
> +that will not override the unset left-aligned bits in the hint address,
> +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 << 38` must be provided. Note that this is 38 due to sv39 userspace
> +ending at :code:`1 << 38` 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 << 47` must be provided.

-- 
~Randy