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

[Darius Rad <darius@bluespec.com>]

On Sat, Aug 09, 2025 at 11:58:24AM +0800, Vivian Wang wrote:
> My previous comment on v1 on prefering clobbering with VS = Initial
> handling aside...
> 
> On 8/8/25 20:36, Darius Rad wrote:
> > 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
> 
> No... Not clobbering does not "leak" anything. If you find that it leaks
> information, please report - that's a bug.
> 

That's why I wrote "may".  If such a bug existed, either now or in the
future, clobbering here would limit the scope of it.  But this may not be
important enough to mention.

> > 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.
> 
> "Robust" is too vague... I don't think this word is helpful for anyone
> trying to understand what this does.
> 
> Vivian "dramforever" Wang
> 
> >
> >> +	  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
> 
> 
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