linux-kernel - Re: [PATCH RT] arm64: fpsimd: use a local_lock() in addition to local_bh

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Message-ID: <20180518124605.GA13470@e103592.cambridge.arm.com>
Date:   Fri, 18 May 2018 13:46:36 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     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
Subject: Re: [PATCH RT] arm64: fpsimd: use a local_lock() in addition to
 local_bh_disable()

On Thu, May 17, 2018 at 07:19:43PM +0100, Dave Martin wrote:

[...]

> kernel_neon_begin() could then do
> 
> 	local_fpsimd_context_lock();
> 
> 	/* ... */
> 
> 	preempt_disable();
> 	local_unlock(fpsimd_context_lock);
> 
> ... with the following in kernel_neon_end():
> 
> 	local_unlock(fpsimd_lock);
> 	preempt_enable();
> 
> 
> If kernel-mode NEON was considered harmful to RT due to the context
> switch overheads, then the above might be overkill.  SVE will be worse
> in that regard, and also needs thinking about at some point -- I've not
> looked at if from the RT angle at all.

Hmmm, !KERNEL_MODE_NEON breaks EFI, so this probably does want looking
at.  Ard's recent rework to enable voluntary preemption the crypto
backends for arm64 [1] should reduce the fpsimd_lock blackouts, but it
still depends on the backends playing nice.

Cheers
---Dave

[1] [PATCH resend 00/10] crypto: arm64 - play nice with CONFIG_PREEMPT
lists.infradead.org/pipermail/linux-arm-kernel/2018-April/574819.html