On Tue, 2024-01-30 at 11:04 -0800, Charlie Jenkins wrote:
> On riscv it is guaranteed that the address returned by mmap is less
> than
> the hint address. Allow mmap to return an address all the way up to
> addr, if provided, rather than just up to the lower address space.
>
> This provides a performance benefit as well, allowing mmap to exit
> after
> checking that the address is in range rather than searching for a
> valid
> address.
>
> It is possible to provide an address that uses at most the same
> number
> of bits, however it is significantly more computationally expensive
> to
> provide that number rather than setting the max to be the hint
> address.
> There is the instruction clz/clzw in Zbb that returns the highest set
> bit
> which could be used to performantly implement this, but it would
> still
> be slower than the current implementation. At worst case, half of the
> address would not be able to be allocated when a hint address is
> provided.
>
> Signed-off-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
> ---
> arch/riscv/include/asm/processor.h | 22 +++++++++-------------
> 1 file changed, 9 insertions(+), 13 deletions(-)
>
> diff --git a/arch/riscv/include/asm/processor.h
> b/arch/riscv/include/asm/processor.h
> index f19f861cda54..5d966ae81a58 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -22,14 +22,12 @@
> ({ \
> unsigned long
> mmap_end; \
> typeof(addr) _addr = (addr); \
> - if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) &&
> is_compat_task())) \
> - mmap_end = STACK_TOP_MAX; \
> - else if ((_addr) >= VA_USER_SV57) \
> - mmap_end = STACK_TOP_MAX; \
> - else if ((((_addr) >= VA_USER_SV48)) && (VA_BITS >=
> VA_BITS_SV48)) \
> - mmap_end = VA_USER_SV48; \
> + if ((_addr) == 0 || \
> + (IS_ENABLED(CONFIG_COMPAT) && is_compat_task()) || \
> + ((_addr + len) > BIT(VA_BITS -
> 1))) \
> + mmap_end = STACK_TOP_MAX \
> else \
> - mmap_end = VA_USER_SV39; \
> + mmap_end = (_addr + len); \
> mmap_end; \
> })
>
> @@ -39,14 +37,12 @@
> typeof(addr) _addr = (addr); \
> typeof(base) _base = (base); \
> unsigned long rnd_gap = DEFAULT_MAP_WINDOW - (_base); \
> - if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) &&
> is_compat_task())) \
> + if ((_addr) == 0 || \
> + (IS_ENABLED(CONFIG_COMPAT) && is_compat_task()) || \
> + ((_addr + len) > BIT(VA_BITS -
> 1))) \
> mmap_base = (_base); \
> - else if (((_addr) >= VA_USER_SV57) && (VA_BITS >=
> VA_BITS_SV57)) \
> - mmap_base = VA_USER_SV57 - rnd_gap; \
> - else if ((((_addr) >= VA_USER_SV48)) && (VA_BITS >=
> VA_BITS_SV48)) \
> - mmap_base = VA_USER_SV48 - rnd_gap; \
> else \
> - mmap_base = VA_USER_SV39 - rnd_gap; \
> + mmap_base = (_addr + len) - rnd_gap; \
Please mind that rnd_gap can be non-zero, in this case, the map will
fail. It will be better to let mmap_base = min((_addr + len), (base) +
TASK_SIZE - DEFAULT_MAP_WINDOW) .
> mmap_base; \
> })
>
>
