Re: [PATCH v1] RISC-V: clarify what some RISCV_ISA* config options do

[Andrew Jones]

On Thu, Apr 18, 2024 at 03:21:01PM +0100, Conor Dooley wrote:
> From: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> 
> During some discussion on IRC yesterday and on Pu's bpf patch [1]
> I noticed that these RISCV_ISA* Kconfig options are not really clear
> about their implications. Many of these options have no impact on what
> userspace is allowed to do, for example an application can use Zbb
> regardless of whether or not the kernel does. Change the help text to
> try and clarify whether or not an option affects just the kernel, or
> also userspace. None of these options actually control whether or not an
> extension is detected dynamically as that's done regardless of Kconfig
> options, so drop any text that implies the option is required for
> dynamic detection, rewording them as "do x when y is detected".
> 
> Link: https://lore.kernel.org/linux-riscv/20240328-ferocity-repose-c554f75a676c@spud/ [1]
> Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
> Signed-off-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> ---
> 
> Vector copy-paste-o fixed, correct spelling of optimisations kept.
> 
> CC: Samuel Holland <samuel.holland@xxxxxxxxxx>
> CC: Pu Lehui <pulehui@xxxxxxxxxxxxxxx>
> CC: Björn Töpel <bjorn@xxxxxxxxxx>
> CC: Paul Walmsley <paul.walmsley@xxxxxxxxxx>
> CC: Palmer Dabbelt <palmer@xxxxxxxxxxx>
> CC: linux-riscv@xxxxxxxxxxxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  arch/riscv/Kconfig | 28 +++++++++++++++-------------
>  1 file changed, 15 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 6d64888134ba..c3a7793b0a7c 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -503,8 +503,8 @@ config RISCV_ISA_SVNAPOT
>  	depends on RISCV_ALTERNATIVE
>  	default y
>  	help
> -	  Allow kernel to detect the Svnapot ISA-extension dynamically at boot
> -	  time and enable its usage.
> +	  Add support for the Svnapot ISA-extension when it is detected by
> +	  the kernel at boot.

I'm not sure we need the 'by the kernel', since I guess that's implied by
being in a Kconfig help text, but either way is fine by me.

>  
>  	  The Svnapot extension is used to mark contiguous PTEs as a range
>  	  of contiguous virtual-to-physical translations for a naturally
> @@ -522,9 +522,9 @@ config RISCV_ISA_SVPBMT
>  	depends on RISCV_ALTERNATIVE
>  	default y
>  	help
> -	   Adds support to dynamically detect the presence of the Svpbmt
> -	   ISA-extension (Supervisor-mode: page-based memory types) and
> -	   enable its usage.
> +	   Add support for the Svpbmt ISA-extension (Supervisor-mode:
> +	   page-based memory types) when it is detected by the kernel at
> +	   boot.

Same 'by the kernel' drop suggestion.

>  
>  	   The memory type for a page contains a combination of attributes
>  	   that indicate the cacheability, idempotency, and ordering
> @@ -543,14 +543,15 @@ config TOOLCHAIN_HAS_V
>  	depends on AS_HAS_OPTION_ARCH
>  
>  config RISCV_ISA_V
> -	bool "VECTOR extension support"
> +	bool "Vector extension support"
>  	depends on TOOLCHAIN_HAS_V
>  	depends on FPU
>  	select DYNAMIC_SIGFRAME
>  	default y
>  	help
>  	  Say N here if you want to disable all vector related procedure
> -	  in the kernel.
> +	  in the kernel. Without this option enabled, neither the kernel nor
> +	  userspace may use vector.
>  
>  	  If you don't know what to do here, say Y.
>  
> @@ -608,8 +609,8 @@ config RISCV_ISA_ZBB
>  	depends on RISCV_ALTERNATIVE
>  	default y
>  	help
> -	   Adds support to dynamically detect the presence of the ZBB
> -	   extension (basic bit manipulation) and enable its usage.
> +	   Add support for enabling optimisations in the kernel when the
> +	   Zbb extension is detected at boot.
>  
>  	   The Zbb extension provides instructions to accelerate a number
>  	   of bit-specific operations (count bit population, sign extending,
> @@ -625,9 +626,9 @@ config RISCV_ISA_ZICBOM
>  	select RISCV_DMA_NONCOHERENT
>  	select DMA_DIRECT_REMAP
>  	help
> -	   Adds support to dynamically detect the presence of the ZICBOM
> -	   extension (Cache Block Management Operations) and enable its
> -	   usage.
> +	   Add support for the Zicbom extension (Cache Block Management
> +	   Operations) and enable its use in the kernel when it is detected
> +	   at boot.
>  
>  	   The Zicbom extension can be used to handle for example
>  	   non-coherent DMA support on devices that need it.
> @@ -686,7 +687,8 @@ config FPU
>  	default y
>  	help
>  	  Say N here if you want to disable all floating-point related procedure
> -	  in the kernel.
> +	  in the kernel. Without this option enabled, neither the kernel nor
> +	  userspace may use floating-point procedures.
>  
>  	  If you don't know what to do here, say Y.
>

Zicboz could also use some clarification, right? Or is the fact that
RISCV_ISA_ZICBOZ enables the use in both the kernel and userspace the
reason "Enable the use of the Zicboz extension (cbo.zero instruction)
when available." looks sufficient? Maybe Zicboz should follow the
"Say N here if..." pattern of V and FPU?

Thanks,
drew