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 <20240129-use_mmap_hint_address-v1-1-4c74da813ba1@rivosinc.com>
Date: Mon, 29 Jan 2024 20:53:31 -0500
From: "Stefan O'Rear" <sorear@fastmail.com>
To: "Charlie Jenkins" <charlie@rivosinc.com>,
 "Alexandre Ghiti" <alexghiti@rivosinc.com>,
 "Paul Walmsley" <paul.walmsley@sifive.com>,
 "Palmer Dabbelt" <palmer@dabbelt.com>, "Albert Ou" <aou@eecs.berkeley.edu>,
 "Shuah Khan" <shuah@kernel.org>, "Jonathan Corbet" <corbet@lwn.net>,
 "Yangyu Chen" <cyy@cyyself.name>
Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
 linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH 1/3] riscv: mm: Use hint address in mmap if available
Content-Type: text/plain

On Mon, Jan 29, 2024, at 7:37 PM, 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@rivosinc.com>
> ---
>  arch/riscv/include/asm/processor.h | 21 ++++++++-------------
>  1 file changed, 8 insertions(+), 13 deletions(-)
>
> diff --git a/arch/riscv/include/asm/processor.h 
> b/arch/riscv/include/asm/processor.h
> index f19f861cda54..f3ea5166e3b2 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -22,14 +22,11 @@
>  ({								\
>  	unsigned long mmap_end;					\
>  	typeof(addr) _addr = (addr);				\
> -	if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) && is_compat_task())) \
> -		mmap_end = STACK_TOP_MAX;			\

Setting mmap_end in the no-hint case seems to have been lost?

-s

> -	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)))		\
>  	else							\
> -		mmap_end = VA_USER_SV39;			\
> +		mmap_end = (_addr + len);			\
>  	mmap_end;						\
>  })
> 
> @@ -39,14 +36,12 @@
>  	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;		\
>  	mmap_base;						\
>  })
> 
>
> -- 
> 2.43.0
>
>
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