Re: [PATCH RT] arm*: disable NEON in kernel mode

[Sebastian Andrzej Siewior]

On 2017-12-01 14:18:28 [+0000], Mark Rutland wrote:
> [Adding Ard, who wrote the NEON crypto code]
> 
> On Fri, Dec 01, 2017 at 02:45:06PM +0100, Sebastian Andrzej Siewior wrote:
> > +arm folks, to let you know
> > 
> > On 2017-12-01 11:43:32 [+0100], To linux-rt-users@xxxxxxxxxxxxxxx wrote:
> > > NEON in kernel mode is used by the crypto algorithms and raid6 code.
> > > While the raid6 code looks okay, the crypto algorithms do not: NEON
> > > is enabled on first invocation and may allocate/free/map memory before
> > > the NEON mode is disabled again.
> 
> Could you elaborate on why this is a problem?
> 
> I guess this is because kernel_neon_{begin,end}() disable preemption?
> 
> ... is this specific to RT?

It is RT specific, yes. One thing are the unbounded latencies since
everything in this preempt_disable section can take time depending on
the size of the request.
The other thing is code like in
  arch/arm64/crypto/aes-ce-ccm-glue.c:ccm_encrypt()

where within this preempt_disable() section skcipher_walk_done() is
invoked. That function can allocate/free/map memory which is okay for
!RT but is not for RT. I tried to break those loops for x86 [0] and I
simply didn't had the time to do the same for ARM. I am aware that
store/restore of the NEON registers (as SSE and AVX) is expensive and
doing a lot of operations in one go is desired. So for x86 I would want
to do some benchmarks and come up with some numbers based on which I
can argue with people one way or another depending on how much it hurts
and how long preemption can be disabled.

[0] https://www.spinics.net/lists/kernel/msg2663115.html

> Thanks,
> Mark.

Sebastian