[RFC PATCH v3 0/4] Simplify kernel-mode NEON

[Ard Biesheuvel]

On 31 May 2017 at 10:08, Dave Martin <Dave.Martin at arm.com> wrote:
> On Wed, May 31, 2017 at 08:41:01AM +0000, Ard Biesheuvel wrote:
>> On 30 May 2017 at 18:02, Dave Martin <Dave.Martin at arm.com> wrote:
>> > On Thu, May 25, 2017 at 07:24:57PM +0100, Dave Martin wrote:
>> >> This series aims to simplify kernel-mode NEON.
>> >
>> > Hi Ard, do you have any further comments on this series?
>> >
>> > I'd like to have it finalised as far as possible (modulo minor tweaks
>> > and bugfixes) so that I can port the SVE patches on top of it.
>> >
>> > Also, how do you think we should handle merging of this change?  There's
>> > a flag-day issue here, since the kernel_mode_neon() API is being changed
>> > in an incompatible way.
>> >
>>
>> I think the patches look fine now. The best way to merge these imo is
>> to start with the changes in the clients, i.e., add an arm64 specific
>> asm/simd.h that defines may_use_simd() as { return true; }, update all
>> the crypto code with the fallbacks, and put this stuff on top of that.
>
> Yes, that sounds feasible.
>
> Something like [1] below?  Either way, it probably makes sense for that
> stub function to be added by your series.
>

Pretty much, yeah. But don't forget to remove simd.h from
arch/arm64/include/asm/Kbuild

>> That way, there is a small window where the 'hint' is interpreted
>> differently in the sha256 code, but apart from that, we should be
>> bisection proof without a flag day AFAICT.
>>
>> BTW I got my ZD1211 working on my MacchiatioBin board. The performance
>> is terrible, but that should not matter: if I can saturate a CPU doing
>
> Do you mean that my series causes a performance regression here, or is
> the performance terrible anyway?
>

No, the performance is terrible, which shouldn't matter per se, but it
would be nice if the load induced by the mac80211 were visible in
'top' as wait, sys or whatever-it-is-called time. Currently, the 3
Mbit/s throughput combined with the 2.2 cycles per byte performance of
the AES-CCM code makes the code unnoticeable.

>> NEON from userland and/or kernel process context, the softirq
>> interruptions by the mac80211 code should exercise the updated code
>> paths. I haven't tried that yet: let me get the code changes out
>> today, so you can put your stuff on top. Then we can give it a good
>> spin.
>
> That would be great, thanks.
>

I have updated my branch here:
https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=kernel-mode-neon

I removed all kernel_neon_begin_partial() invocations as well.

-- 
Ard.