[PATCH v3 21/28] arm64/sve: Add sysctl to set the default vector length for new processes

[Catalin Marinas]

On Tue, Oct 10, 2017 at 07:38:38PM +0100, Dave P Martin wrote:
> Because of the effect of SVE on the size of the signal frame, the
> default vector length used for new processes involves a tradeoff
> between performance of SVE-enabled software on the one hand, and
> reliability of non-SVE-aware software on the other hand.
> 
> For this reason, the best choice depends on the repertoire of
> userspace software in use and is thus best left up to distro
> maintainers, sysadmins and developers.
> 
> If CONFIG_SYSCTL is enabled, this patch exposes the default vector
> length in /proc/sys/abi/sve_default_vector_length, where boot
> scripts or the adventurous can poke it.
> 
> In common with other arm64 ABI sysctls, this control is currently
> global: setting it requires CAP_SYS_ADMIN in the root user
> namespace, but the value set is effective for subsequent execs in
> all namespaces.  The control only affects _new_ processes, however:
> changing it does not affect the vector length of any existing
> process.
> 
> The intended usage model is that if userspace is known to be fully
> SVE-tolerant (or a developer is curious to find out) then init
> scripts can crank this up during startup.
> 
> Signed-off-by: Dave Martin <Dave.Martin at arm.com>
> Reviewed-by: Alex Bennée <alex.bennee at linaro.org>

Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>