linux-kernel - Re: [PATCH 2/2] docs: riscv: hwprobe: Clarify misaligned keys are values not bitmasks

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Message-ID: <CALs-HssGcNso6vTfbcsiWX1h_46jgDDRcEWcfZCTpxXYnubcng@mail.gmail.com>
Date: Tue, 21 May 2024 11:36:06 -0700
From: Evan Green <evan@...osinc.com>
To: Yangyu Chen <cyy@...self.name>
Cc: linux-riscv@...ts.infradead.org, Elliott Hughes <enh@...gle.com>, 
	Charlie Jenkins <charlie@...osinc.com>, Jonathan Corbet <corbet@....net>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Clément Léger <cleger@...osinc.com>, 
	Conor Dooley <conor.dooley@...rochip.com>, Andrew Jones <ajones@...tanamicro.com>, 
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/2] docs: riscv: hwprobe: Clarify misaligned keys are
 values not bitmasks

On Sat, May 18, 2024 at 9:00 AM Yangyu Chen <cyy@...self.name> wrote:
>
> The original documentation says hwprobe keys are bitmasks, but actually,
> they are values. This patch clarifies this to avoid confusion.
>
> Signed-off-by: Yangyu Chen <cyy@...self.name>

Hm, we also have this problem in the code, since
hwprobe_key_is_bitmask() returns true for KEY_CPUPERF_0. This results
in wrong information being returned for queries using the WHICH_CPU
flag. If usermode asked for the set of CPUs that was specifically SLOW
or EMULATED, the returned cpuset would also include cpus that were
FAST. I believe all other queries are okay.

The one-liner fix is to just not return true for that key in
hwprobe_key_is_bitmask(). But that's technically user-visible: if some
software relied on the buggy behavior of FAST cpus being swept up in
the query for SLOW or EMULATED cpus, this change would expose that.
The grownups-eat-their-vegetables thing to do would be to define a new
key that returns this same value, but doesn't return true in
hwprobe_key_is_bitmask(). What do people think?

-Evan

> ---
>  Documentation/arch/riscv/hwprobe.rst | 31 ++++++++++++++++------------
>  1 file changed, 18 insertions(+), 13 deletions(-)
>
> diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst
> index 239be63f5089..4abfa3f9fe44 100644
> --- a/Documentation/arch/riscv/hwprobe.rst
> +++ b/Documentation/arch/riscv/hwprobe.rst
> @@ -188,25 +188,30 @@ The following keys are defined:
>         manual starting from commit 95cf1f9 ("Add changes requested by Ved
>         during signoff")
>
> -* :c:macro:`RISCV_HWPROBE_KEY_CPUPERF_0`: A bitmask that contains performance
> +* :c:macro:`RISCV_HWPROBE_KEY_CPUPERF_0`: A value that contains performance
>    information about the selected set of processors.
>
> -  * :c:macro:`RISCV_HWPROBE_MISALIGNED_UNKNOWN`: The performance of misaligned
> -    scalar accesses is unknown.
> +  * :c:macro:`RISCV_HWPROBE_MISALIGNED_MASK`: The bitmask of the misaligned
> +    access performance field in the value of key `RISCV_HWPROBE_KEY_CPUPERF_0`.
>
> -  * :c:macro:`RISCV_HWPROBE_MISALIGNED_EMULATED`: Misaligned scalar accesses are
> -    emulated via software, either in or below the kernel.  These accesses are
> -    always extremely slow.
> +    The following values (not bitmasks) in this field are defined:
>
> -  * :c:macro:`RISCV_HWPROBE_MISALIGNED_SLOW`: Misaligned scalar accesses are
> -    slower than equivalent byte accesses.  Misaligned accesses may be supported
> -    directly in hardware, or trapped and emulated by software.
> +    * :c:macro:`RISCV_HWPROBE_MISALIGNED_UNKNOWN`: The performance of misaligned
> +      scalar accesses is unknown.
>
> -  * :c:macro:`RISCV_HWPROBE_MISALIGNED_FAST`: Misaligned scalar accesses are
> -    faster than equivalent byte accesses.
> +    * :c:macro:`RISCV_HWPROBE_MISALIGNED_EMULATED`: Misaligned scalar accesses are
> +      emulated via software, either in or below the kernel.  These accesses are
> +      always extremely slow.
>
> -  * :c:macro:`RISCV_HWPROBE_MISALIGNED_UNSUPPORTED`: Misaligned scalar accesses
> -    are not supported at all and will generate a misaligned address fault.
> +    * :c:macro:`RISCV_HWPROBE_MISALIGNED_SLOW`: Misaligned scalar accesses are
> +      slower than equivalent byte accesses.  Misaligned accesses may be supported
> +      directly in hardware, or trapped and emulated by software.
> +
> +    * :c:macro:`RISCV_HWPROBE_MISALIGNED_FAST`: Misaligned scalar accesses are
> +      faster than equivalent byte accesses.
> +
> +    * :c:macro:`RISCV_HWPROBE_MISALIGNED_UNSUPPORTED`: Misaligned scalar accesses
> +      are not supported at all and will generate a misaligned address fault.
>
>  * :c:macro:`RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE`: An unsigned int which
>    represents the size of the Zicboz block in bytes.
> --
> 2.43.0
>