This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [PATCH v3 21/28] arm64/sve: Add sysctl to set the default vector length for new processes
- From: Catalin Marinas <catalin dot marinas at arm dot com>
- To: Dave Martin <Dave dot Martin at arm dot com>
- Cc: linux-arm-kernel at lists dot infradead dot org, linux-arch at vger dot kernel dot org, Okamoto Takayuki <tokamoto at jp dot fujitsu dot com>, libc-alpha at sourceware dot org, Ard Biesheuvel <ard dot biesheuvel at linaro dot org>, Szabolcs Nagy <szabolcs dot nagy at arm dot com>, Will Deacon <will dot deacon at arm dot com>, Richard Sandiford <richard dot sandiford at arm dot com>, Alex Bennée <alex dot bennee at linaro dot org>, kvmarm at lists dot cs dot columbia dot edu
- Date: Thu, 12 Oct 2017 18:11:47 +0100
- Subject: Re: [PATCH v3 21/28] arm64/sve: Add sysctl to set the default vector length for new processes
- Authentication-results: sourceware.org; auth=none
- References: <1507660725-7986-1-git-send-email-Dave.Martin@arm.com> <1507660725-7986-22-git-send-email-Dave.Martin@arm.com>
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@arm.com>
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>