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SubjectRe: [PATCH v2 3/7] irqchip/apple-aic: Add Fast IPI support
On Sun, 27 Feb 2022 15:33:54 +0000,
Hector Martin <marcan@marcan.st> wrote:
>
> On 25/02/2022 23.39, Marc Zyngier wrote:
> > On Thu, 24 Feb 2022 13:07:37 +0000,
> >> if (!(pending & irq_bit) &&
> >> - (atomic_read(per_cpu_ptr(&aic_vipi_enable, cpu)) & irq_bit))
> >> - send |= AIC_IPI_SEND_CPU(cpu);
> >> + (atomic_read(per_cpu_ptr(&aic_vipi_enable, cpu)) & irq_bit)) {
> >> + if (static_branch_likely(&use_fast_ipi))
> >> + aic_ipi_send_fast(cpu);
> >
> > OK, this is suffering from the same issue that GICv3 has, which is
> > that memory barriers don't provide order against sysregs. You need a
> > DSB for that, which is a pain. Something like this:
>
> Doesn't the control flow here guarantee the ordering? atomic_read() must
> complete before the sysreg is written since there is a control flow
> dependency, and the prior atomic/barrier dance ensures that read is
> ordered properly with everything that comes before it.

Yes, you're right. Mixing memory ordering and control dependency hurts
my head badly, but hey, why not.

M.

--
Without deviation from the norm, progress is not possible.

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