[PATCH v3 4/5] arm64: crypto: Remove conditional yield logic

[Ard Biesheuvel]

On Mon, 27 Nov 2023 at 14:46, Mark Rutland <mark.rutland at arm.com> wrote:
>
> On Mon, Nov 27, 2023 at 01:23:04PM +0100, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb at kernel.org>
> >
> > Some classes of crypto algorithms (such as skciphers or aeads) have
> > natural yield points, but SIMD based shashes yield the NEON unit
> > manually to avoid causing scheduling blackouts when operating on large
> > inputs.
> >
> > This is no longer necessary now that kernel mode NEON runs with
> > preemption enabled, so remove this logic from the crypto assembler code,
> > along with the macro that implements the TIF_NEED_RESCHED check.
> >
> > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
>
> I definitely want to get rid of all of the voluntary preemption points, but
> IIUC for the moment we need to keep these for PREEMPT_NONE and
> PREEMPT_VOLUNTARY (and consequently for PREEMPT_DYNAMIC). Once the preemption
> rework lands, these should no longer be necessary and can be removed:
>
>   https://lore.kernel.org/lkml/20231107215742.363031-1-ankur.a.arora@oracle.com/
>

Oh, right - yeah, good point.

So until that lands, we could at least simplify cond_yield and go back
to the original logic, given that yielding to a pending softirq will
no longer be necessary. (The original logic does not deal with
softirqs specifically, but relies on the preempt_count() not being
equal to PREEMPT_DISABLE_OFFSET to avoid yielding in sofirq context
unnecessarily)

This also means that only PREEMPT_VOLUNTARY is implicated here -
PREEMPT_NONE only has the yield-to-pending-softirq behavior atm.