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Re: [PATCH v2 26/28] arm64/sve: Add documentation
- From: Alex Bennée <alex dot bennee at linaro dot org>
- To: Dave Martin <Dave dot Martin at arm dot com>
- Cc: linux-arch at vger dot kernel dot org, Mark Rutland <mark dot rutland at arm 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>, Catalin Marinas <catalin dot marinas at arm dot com>, Will Deacon <will dot deacon at arm dot com>, Richard Sandiford <richard dot sandiford at arm dot com>, nd at arm dot com, kvmarm at lists dot cs dot columbia dot edu, linux-arm-kernel at lists dot infradead dot org
- Date: Mon, 09 Oct 2017 15:07:23 +0100
- Subject: Re: [PATCH v2 26/28] arm64/sve: Add documentation
- Authentication-results: sourceware.org; auth=none
- References: <1504198860-12951-1-git-send-email-Dave.Martin@arm.com> <1504198860-12951-27-git-send-email-Dave.Martin@arm.com> <59D7A4AF.40408@arm.com> <20171006173715.GC3611@e103592.cambridge.arm.com> <878tgkbr6m.fsf@linaro.org> <20171009094930.GD3611@e103592.cambridge.arm.com>
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/.
--
Alex Bennée