[RFC] Correct behaviour of irq affinity?

[Rusty Russell <rusty@rustcorp.com.au>]

The effect of setting desc->affinity (ie. from userspace via sysfs) has varied
over time.  In 2.6.27, the 32-bit code anded the value with cpu_online_map,
and both 32 and 64-bit did that anding whenever a cpu was unplugged.

2.6.29 consolidated this into one routine (and fixed hotplug) but introduced
another variation: anding the affinity with cfg->domain.  Is this right, or
should we just set it to what the user said?  Or as now, indicate that we're
restricting it.

If we should change it, here's what the patch looks like against x86 tip
(cpu_mask_to_apicid_and already takes cpu_online_mask into account):

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index 86827d8..30906cd 100644
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/arch/x86/kernel/apic/io_apic.c
@@ -592,10 +592,10 @@ set_desc_affinity(struct irq_desc *desc, const struct cpumask *mask)
 	if (assign_irq_vector(irq, cfg, mask))
 		return BAD_APICID;
 
-	cpumask_and(desc->affinity, cfg->domain, mask);
+	cpumask_copy(desc->affinity, mask);
 	set_extra_move_desc(desc, mask);
 
-	return apic->cpu_mask_to_apicid_and(desc->affinity, cpu_online_mask);
+	return apic->cpu_mask_to_apicid_and(desc->affinity, cfg->domain);
 }
 
 static void