NMI for ARC

[Peter Zijlstra]

On Tue, Nov 17, 2015 at 06:23:21PM +0530, Vineet Gupta wrote:
> On Tuesday 17 November 2015 05:55 PM, Peter Zijlstra wrote:
> 
> > This is assuming you now have these NMIs we talked about earlier. If all
> > you have are regular IRQs this is not possible, for we should be calling
> > ->read() with IRQs disabled.
> > 
> 
> No we don't yet. The first stab at it fell flat on floor.
> 
> The NMI support from hardware is that is it provides different priorities, higher
> one obviously able to interrupt lower one. However instructions like CLRI (disable
> interrupts) will still lock out all interrupts.
> 
> Thus local_irq_save()/restore() and local_irq_enable()/disable() now need to be
> contextual.
> 
>   - When running in prio 0 mode, they only need to enable 0
>   - In prio 1, they need to enable both 0 and 1
> 
> For irq_save()/restore() this is achievable by doing an additional STATUS32 read
> at the time of save and passing that value to restore - so there's an additional
> overhead - but ignoring that for now.
> 
> Bummer is irq_disable()/enable() case: there's need to pass old prio state from
> enable to disabled, so we need some sort of global state tracking - which in case
> of SMP needs to be per cpu.... either keep something hot in a reg or pay the cost
> of additional mem/cache line miss.
> 
> I've not investigated how other arches do that. PPC seems to be using some sort of
> soft irq state anyways.

Yeah, Sparc64 might be a better example, it more closely matches your
hardware. See
arch/sparc/include/asm/irqflags_64.h:arch_local_irq_save().