x86: avoid domains with no usable CPUs

[emaste]

Summary

If we limit the CPUs which will receive interrupts we may have a domain with no usable CPUs. Allow any allowable CPU to be used when requesting a CPU for such a domain.

[emaste]

I was thinking of adding a tunable to have set_interrupt_apic_ids skip APIC IDs > 255. However, this may result in a &cpuset[domain] that has no common entries with &intr_cpus, hence this review. Very happy to hear of other ideas for a usable workaround for this APIC ID > 255 issue.

[emaste]

Or maybe something simpler - if we find an unusable domain fall back to no per-domain lists. D41501

[markj]

sys/x86/x86/intr_machdep.c

587

I think you can replace this loop with domain_usable[i] = CPU_OVERLAP(&cpuset_domain[i], &intr_cpus);.

589

Maybe "CPUs from domain %d cannot service interrupts"? "has no usable CPUs" sounds scary and might confuse users.

[emaste]

sys/x86/x86/intr_machdep.c

587

I think I prefer the D41501 approach, but will apply both of these comments there

[emaste]

feedback from markj in D41500

[emaste]

oops updated wrong review - restore this diff

[jhb]

Hmm, my suggestion over in D41501 is a hybrid of both approaches I think.