Re: [PATCH v2] riscv: Add sysctl to control discard of vstate during syscall

[Darius Rad <darius@bluespec.com>]

On Wed, Aug 06, 2025 at 07:03:28AM -0700, Drew Fustini wrote:
[...]
> 
> diff --git a/Documentation/arch/riscv/vector.rst b/Documentation/arch/riscv/vector.rst
> index 3987f5f76a9deb0824e53a72df4c3bf90ac2bee1..b702c00351617165a4d8897c7df68eadcd2d562e 100644
> --- a/Documentation/arch/riscv/vector.rst
> +++ b/Documentation/arch/riscv/vector.rst
> @@ -134,7 +134,25 @@ processes in form of sysctl knob:
>  3.  Vector Register State Across System Calls
>  ---------------------------------------------
>  
> -As indicated by version 1.0 of the V extension [1], vector registers are
> -clobbered by system calls.
> +Linux adopts the syscall ABI proposed by version 1.0 of the V extension [1],
> +where vector registers are clobbered by system calls. Specifically:
> +
> +    Executing a system call causes all caller-saved vector registers
> +    (v0-v31, vl, vtype) and vstart to become unspecied.
> +

Perhaps:

Clobbering the vector registers may prevent leaking information to user
space and aid in debugging, but can significantly increase system call
latency for some implementations.  [...]

> +However, clobbering the vector registers can significantly increase system call
> +latency for some implementations. To mitigate this performance impact, a sysctl
> +knob is provided that controls whether vector state is always discarded in the
> +syscall path:
> +
> +* /proc/sys/abi/riscv_v_vstate_discard
> +
> +    Valid values are:
> +
> +    * 0: Vector state is not always clobbered in all syscalls
> +    * 1: Mandatory clobbering of vector state in all syscalls
> +
> +    Reading this file returns the current discard behavior. The initial state is
> +    controlled by CONFIG_RISCV_ISA_V_VSTATE_DISCARD.
>  
>  1: https://github.com/riscv/riscv-v-spec/blob/master/calling-convention.adoc
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 36061f4732b7496a9c68a9a10f9959849dc2a95c..7bb8a8513135cbc105bd94d273012486a886f724 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -656,6 +656,16 @@ config RISCV_ISA_V_DEFAULT_ENABLE
>  
>  	  If you don't know what to do here, say Y.
>  
> +config RISCV_ISA_V_VSTATE_DISCARD
> +	bool "Enable Vector state discard by default"
> +	depends on RISCV_ISA_V
> +	default n
> +	help

Perhaps add the following paragraph:

Discarding vector state is more robust, but has negative performance
implications in certain implementations.


> +	  Say Y here if you want to always discard vector state in syscalls.
> +	  Otherwise, userspace has to enable it via the sysctl interface.
> +
> +	  If you don't know what to do here, say N.
> +
>  config RISCV_ISA_V_UCOPY_THRESHOLD
>  	int "Threshold size for vectorized user copies"
>  	depends on RISCV_ISA_V