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

[Darius Rad <darius@bluespec.com>]

On Fri, Jul 18, 2025 at 08:39:13PM -0700, Drew Fustini wrote:
> From: Drew Fustini <dfustini@tenstorrent.com>
> 
> Clobbering the vector registers can significantly increase system call
> latency for some implementations. To mitigate this performance impact, a
> policy mechanism is provided to administrators, distro maintainers, and
> developers to control vector state discard in the form of a sysctl knob:
> 
> /proc/sys/abi/riscv_v_vstate_discard
> 
> Valid values are:
> 
> 0: Do not discard vector state during syscall
> 1: Discard vector state during syscall
> 

Is the intention for this this mean "don't guarantee vector state is
clobbered" or "preserve vector state"?  I suspect it is the former, but the
wording seems unclear.  Additionally, if that's indeed the case, maybe the
documentation should more clearly articulate the tradeoff (performance vs.
security/robustness).

> The initial state is controlled by CONFIG_RISCV_ISA_V_VSTATE_DISCARD.
> 
> Fixes: 9657e9b7d253 ("riscv: Discard vector state on syscalls")
> Signed-off-by: Drew Fustini <dfustini@tenstorrent.com>
> ---
>  Documentation/arch/riscv/vector.rst | 15 +++++++++++++++
>  arch/riscv/Kconfig                  | 10 ++++++++++
>  arch/riscv/include/asm/vector.h     |  4 ++++
>  arch/riscv/kernel/vector.c          | 16 +++++++++++++++-
>  4 files changed, 44 insertions(+), 1 deletion(-)
> 
> I've tested the impact of riscv_v_vstate_discard() on the SiFive X280
> cores [1] in the Tenstorrent Blackhole SoC [2]. The results from the
> Blackhole P100 [3] card show that discarding the vector registers
> increases null syscall latency by 25%.
> 
> The null syscall program [4] executes vsetvli and then calls getppid()
> in a loop. The average duration of getppid() is 198 ns when registers
> are clobbered in riscv_v_vstate_discard(). The average duration drops
> to 149 ns when riscv_v_vstate_discard() skips clobbering the registers
> as result of riscv_v_vstate_discard being set to 0.
> 
> $ sudo sysctl abi.riscv_v_vstate_discard=1
> abi.riscv_v_vstate_discard = 1
> 
> $ ./null_syscall --vsetvli
> vsetvli complete
>  iterations: 1000000000
>    duration: 198 seconds
> avg latency: 198.73 ns
> 
> $ sudo sysctl abi.riscv_v_vstate_discard=0
> abi.riscv_v_vstate_discard = 0
> 
> $ ./null_syscall --vsetvli
> vsetvli complete
>  iterations: 1000000000
>    duration: 149 seconds
> avg latency: 149.89 ns
> 
> I'm testing on the tt-blackhole-v6.16-rc1_vstate_discard [5] branch that
> has 13 patches, including this one, on top of v6.16-rc1. Most are simple
> yaml patches for dt bindings along with dts files and a bespoke network
> driver. I don't think the other patches are relevant to this discussion.
> 
> This patch applies clean on its own to riscv/for-next and next-20250718.
> 
> [1] https://www.sifive.com/cores/intelligence-x200-series
> [2] https://tenstorrent.com/en/hardware/blackhole
> [3] https://github.com/tenstorrent/tt-bh-linux
> [4] https://gist.github.com/tt-fustini/ab9b217756912ce75522b3cce11d0d58
> [5] https://github.com/tenstorrent/linux/tree/tt-blackhole-v6.16-rc1_vstate_discard
> 
> diff --git a/Documentation/arch/riscv/vector.rst b/Documentation/arch/riscv/vector.rst
> index 3987f5f76a9d..1edbce436015 100644
> --- a/Documentation/arch/riscv/vector.rst
> +++ b/Documentation/arch/riscv/vector.rst
> @@ -137,4 +137,19 @@ processes in form of sysctl knob:
>  As indicated by version 1.0 of the V extension [1], vector registers are
>  clobbered by system calls.
>  
> +Clobbering the vector registers can significantly increase system call latency
> +for some implementations. To mitigate the performance impact, a policy mechanism
> +is provided to the administrators, distro maintainers, and developers to control
> +the vstate discard in the form of a sysctl knob:
> +
> +* /proc/sys/abi/riscv_v_vstate_discard
> +
> +    Valid values are:
> +
> +    * 0: Do not discard vector state during syscall
> +    * 1: Discard vector state during syscall
> +
> +    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 0aeee50da016..c0039f21d1f0 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -655,6 +655,16 @@ 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
> +	  Say Y here if you want to enable Vector state discard on syscall.
> +	  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
> diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
> index 45c9b426fcc5..77991013216b 100644
> --- a/arch/riscv/include/asm/vector.h
> +++ b/arch/riscv/include/asm/vector.h
> @@ -40,6 +40,7 @@
>  	_res;								\
>  })
>  
> +extern bool riscv_v_vstate_discard_ctl;
>  extern unsigned long riscv_v_vsize;
>  int riscv_v_setup_vsize(void);
>  bool insn_is_vector(u32 insn_buf);
> @@ -270,6 +271,9 @@ static inline void __riscv_v_vstate_discard(void)
>  {
>  	unsigned long vl, vtype_inval = 1UL << (BITS_PER_LONG - 1);
>  
> +	if (READ_ONCE(riscv_v_vstate_discard_ctl) == 0)
> +		return;
> +
>  	riscv_v_enable();
>  	if (has_xtheadvector())
>  		asm volatile (THEAD_VSETVLI_T4X0E8M8D1 : : : "t4");
> diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
> index 184f780c932d..7a4c209ad337 100644
> --- a/arch/riscv/kernel/vector.c
> +++ b/arch/riscv/kernel/vector.c
> @@ -26,6 +26,7 @@ static struct kmem_cache *riscv_v_user_cachep;
>  static struct kmem_cache *riscv_v_kernel_cachep;
>  #endif
>  
> +bool riscv_v_vstate_discard_ctl = IS_ENABLED(CONFIG_RISCV_ISA_V_VSTATE_DISCARD);
>  unsigned long riscv_v_vsize __read_mostly;
>  EXPORT_SYMBOL_GPL(riscv_v_vsize);
>  
> @@ -307,11 +308,24 @@ static const struct ctl_table riscv_v_default_vstate_table[] = {
>  	},
>  };
>  
> +static const struct ctl_table riscv_v_vstate_discard_table[] = {
> +	{
> +		.procname       = "riscv_v_vstate_discard",
> +		.data           = &riscv_v_vstate_discard_ctl,
> +		.maxlen         = sizeof(riscv_v_vstate_discard_ctl),
> +		.mode           = 0644,
> +		.proc_handler   = proc_dobool,
> +	},
> +};
> +
>  static int __init riscv_v_sysctl_init(void)
>  {
> -	if (has_vector() || has_xtheadvector())
> +	if (has_vector() || has_xtheadvector()) {
>  		if (!register_sysctl("abi", riscv_v_default_vstate_table))
>  			return -EINVAL;
> +		if (!register_sysctl("abi", riscv_v_vstate_discard_table))
> +			return -EINVAL;
> +	}
>  	return 0;
>  }
>  
> -- 
> 2.34.1
> 
> 
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