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Date: Fri, 1 Dec 2017 18:14:58 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Sebastian Andrzej Siewior <bigeasy@...utronix.de> Cc: Mark Rutland <mark.rutland@....com>, linux-rt-users@...r.kernel.org, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>, tglx@...utronix.de, linux-arm-kernel@...ts.infradead.org, ard.biesheuvel@...aro.org Subject: Re: [PATCH RT] arm*: disable NEON in kernel mode On Fri, Dec 01, 2017 at 03:36:48PM +0100, Sebastian Andrzej Siewior wrote: > 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@...r.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. Well, PREEMPT cares about that too.
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