Re: [PATCH v2 26/28] arm64/sve: Add documentation

Alex Bennée
2017-10-09T14:07:23Z

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

> On Mon, Oct 09, 2017 at 10:34:25AM +0100, Alex Bennée wrote:
>>
>> Dave Martin <Dave.Martin@arm.com> writes:
>>
>> > On Fri, Oct 06, 2017 at 04:43:43PM +0100, Szabolcs Nagy wrote:
>> >> On 31/08/17 18:00, Dave Martin wrote:
>> >> > +9. System runtime configuration
>> >> > +--------------------------------
>> >> > +
>> >> > +* To mitigate the ABI impact of expansion of the signal frame, a policy
>> >> > + mechanism is provided for administrators, distro maintainers and developers
>> >> > + to set the default vector length for userspace processes:
>> >> > +
>> >> > +/proc/cpu/sve_default_vector_length
>> >>
>> >>
>> >> elsewhere in the patch series i see
>> >>
>> >> /proc/sys/abi/sve_default_vector_length
>> >>
>> >> is this supposed to be the same?
>> >
>> > Good spot, thanks!
>> >
>> > /proc/cpu/ was the old location: they should both say /proc/abi/.
>> > I'll fix it.
>>
>> Isn't /sys (or rather sysfs) the preferred location for modern control
>> knobs that mirror the kernels object model or is SVE a special case for
>> extending /proc?
>
> I couldn't figure out which kernel object this maps to. There's no
> device, no driver. This isn't even per-cpu.

Hmm I can see:

/sys/devices/system/cpu

On both my x86 and arm64 systems - but I guess this is more ABIish than
CPU feature related.

> sysctl is already used for similar knobs to this one, so I followed that
> precedent -- though if someone argues strongly enough it could be
> changed.
>
> Are there already examples of arch controls like this in sysfs? I
> wasn't aware of any, but I didn't look all that hard...

Given the paucity of the /proc/sys/abi on both systems I guess this sort
of knob is rare enough that people haven't expressed a strong preference
for sysfs here. I have no objection to staying with /proc/sys/abi/.