On Tuesday 16 June 2015 02:37 PM, Peter Zijlstra wrote: > On Tue, Jun 16, 2015 at 05:37:40AM +0000, Vineet Gupta wrote: >> > On Monday 15 June 2015 09:55 PM, Peter Zijlstra wrote: >>> > > On Tue, Jun 09, 2015 at 05:49:28PM +0530, Vineet Gupta wrote: >>>> > >> + if (arc_pmu->has_interrupts) { >>>> > >> + int irq = platform_get_irq(pdev, 0); >>> > > Hmm, so you're requesting a regular interrupt. >>> > > >>> > > I see your architecture has IRQ priorities, could you play games and >>> > > create NMIs using those? >>> > > >>> > > For example, never mask L1 (assuming that's the highest priority) and >>> > > treat that as an NMI. >> > >> > I've had this idea before, however, while ARCv2 provides hardware interrupt >> > priorities, we really can't implement true NMI, because CLRI / SETI used at >> > backend of loal_irq_save() / restore() impact all priorities (statsu32 >> > register >> > has a global enable interrupt bit which these wiggle). So e.g. a >> > spin_lock_irqsave() will lock out even the perf interrupt. > Hmm, bugger. I (of course) only looked at the kernel source, since that > is all I have, and the current arch/arc/ frobs with those two En bits in > status32. > > So arcv2 changed all that, shame.
Turns out that it is possible to implement NMI on ARCv2 in a pretty straightforward way. Our RTOS Guru, Chuck, told me off list, that instead of using CLRI / SETI, we can use SETI with different args which would keep the stat32.IE enabled all the times, but wiggle the stat32.E[ ] to change the intr prio level, effectively locking out only lower prio interrupts in any local_irq_save() / restore() region. But isn't this defying the irq disable/enable semantics and could lead to potential breach of *some* critical section. Neverthless, doing this requires some more changes in ARCv2 support code - so for now we will go with the normal interrupts and later bolt on the NMI emulation. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/