x86: Correct behaviour of irq affinity

Impact: get correct smp_affinity as user requested

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.

We should just set it to what the user said - if possible.

(cpu_mask_to_apicid_and already takes cpu_online_mask into account)

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <49C94DDF.2010703@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/apic/io_apic.c

@@ -594,9 +594,10 @@ set_desc_affinity(struct irq_desc *desc, const struct cpumask *mask)
 
 	/* check that before desc->addinity get updated */
 	set_extra_move_desc(desc, mask);
-	cpumask_and(desc->affinity, cfg->domain, mask);
 
-	return apic->cpu_mask_to_apicid_and(desc->affinity, cpu_online_mask);
+	cpumask_copy(desc->affinity, mask);
+
+	return apic->cpu_mask_to_apicid_and(desc->affinity, cfg->domain);
 }
 
 static void