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SubjectRe: [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

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