Messages in this thread | | | Date | Fri, 1 Dec 2017 15:36:48 +0100 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH RT] arm*: disable NEON in kernel mode |
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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@vger.kernel.org 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
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