[PATCH v2 21/28] arm64/sve: Add sysctl to set the default vector length for new processes

[Alex Bennée]

Dave Martin <Dave.Martin at arm.com> writes:

> 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>
> Cc: Alex Bennée <alex.bennee at linaro.org>

Reviewed-by: Alex Bennée <alex.bennee at linaro.org>

>
> ---
>
> Changes since v1
> ----------------
>
> Changes requested by Alex Bennée:
>
> * Thin out BUG_ON()s:
> Redundant BUG_ON()s and ones that just check invariants are removed.
> Important sanity-checks are migrated to WARN_ON()s, with some
> minimal best-effort patch-up code.
> ---
>  arch/arm64/kernel/fpsimd.c | 62 +++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 61 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 42e8331..b430ee0 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -34,6 +34,7 @@
>  #include <linux/sched/signal.h>
>  #include <linux/signal.h>
>  #include <linux/slab.h>
> +#include <linux/sysctl.h>
>
>  #include <asm/fpsimd.h>
>  #include <asm/cputype.h>
> @@ -244,6 +245,65 @@ static unsigned int find_supported_vector_length(unsigned int vl)
>  	return sve_vl_from_vq(bit_to_vq(bit));
>  }
>
> +#ifdef CONFIG_SYSCTL
> +
> +static int sve_proc_do_default_vl(struct ctl_table *table, int write,
> +				  void __user *buffer, size_t *lenp,
> +				  loff_t *ppos)
> +{
> +	int ret;
> +	int vl = sve_default_vl;
> +	struct ctl_table tmp_table = {
> +		.data = &vl,
> +		.maxlen = sizeof(vl),
> +	};
> +
> +	ret = proc_dointvec(&tmp_table, write, buffer, lenp, ppos);
> +	if (ret || !write)
> +		return ret;
> +
> +	/* Writing -1 has the special meaning "set to max": */
> +	if (vl == -1) {
> +		/* Fail safe if sve_max_vl wasn't initialised */
> +		if (WARN_ON(!sve_vl_valid(sve_max_vl)))
> +			vl = SVE_VL_MIN;
> +		else
> +			vl = sve_max_vl;
> +
> +		goto chosen;
> +	}
> +
> +	if (!sve_vl_valid(vl))
> +		return -EINVAL;
> +
> +	vl = find_supported_vector_length(vl);
> +chosen:
> +	sve_default_vl = vl;
> +	return 0;
> +}
> +
> +static struct ctl_table sve_default_vl_table[] = {
> +	{
> +		.procname	= "sve_default_vector_length",
> +		.mode		= 0644,
> +		.proc_handler	= sve_proc_do_default_vl,
> +	},
> +	{ }
> +};
> +
> +static int __init sve_sysctl_init(void)
> +{
> +	if (system_supports_sve())
> +		if (!register_sysctl("abi", sve_default_vl_table))
> +			return -EINVAL;
> +
> +	return 0;
> +}
> +
> +#else /* ! CONFIG_SYSCTL */
> +static int __init sve_sysctl_init(void) { return 0; }
> +#endif /* ! CONFIG_SYSCTL */
> +
>  #define ZREG(sve_state, vq, n) ((char *)(sve_state) +		\
>  	(SVE_SIG_ZREG_OFFSET(vq, n) - SVE_SIG_REGS_OFFSET))
>
> @@ -1030,6 +1090,6 @@ static int __init fpsimd_init(void)
>  	if (!(elf_hwcap & HWCAP_ASIMD))
>  		pr_notice("Advanced SIMD is not implemented\n");
>
> -	return 0;
> +	return sve_sysctl_init();
>  }
>  late_initcall(fpsimd_init);


--
Alex Bennée