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

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Message-ID: <20171201171458.3a2acsltiq54gvuu@hirez.programming.kicks-ass.net>
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.