On Mon, Jul 06, 2015 at 04:56:09PM +0300, Max Filippov wrote:
> On Mon, Jul 6, 2015 at 4:47 PM, Peter Zijlstra <[email protected]> wrote:
> > On Mon, Jul 06, 2015 at 04:32:48PM +0300, Max Filippov wrote:
> >> +static int __init xtensa_pmu_init(void)
> >> +{
> >> + int ret;
> >> + int irq = irq_create_mapping(NULL, XCHAL_PROFILING_INTERRUPT);
> >
> > Does this platform have interrupt priorities which you can partially
> > mask in order to create NMI like behaviour?
>
> Not sure what you mean by "NMI like".
There's a number of archs where we implement NMIs by having
local_irq_disable() only disable part of the interrupt priority range
and making sure all 'normal' IRQs are mapped in that priority range.
We then map our NMI handlers to a priority above the 'normal' range,
such that these interrupts can indeed happen when interrupts are
'disabled.
See for example:
b4f4372f96e0 ("sparc64: Make %pil level 15 a pseudo-NMI.")
0c25e9e6cbe7 ("sparc64: Adjust __raw_local_irq_save() to cooperate in NMIs.")
c011f80ba091 ("sparc64: Add some more commentary to __raw_local_irq_save()")
> Interrupt priorities are fixed in the current xtensa architecture, and
> we can in theory mask certain level and below, but practically we
> always mask all low- and medium- level interrupts.
>
> Also we currently can't have handlers for high priority interrupts written in
> C.
Why not? Surely this can be cured with an assembly stub?
The advantage of having NMIs is that profiling information for the
kernel becomes much more useful. Without this local_irq_enable() will be
a very 'hot' function.
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