[PATCH 2/16] xics: update default_server during migrate_irqs_away

[Milton Miller <miltonm@bga.com>]

Currently, every time we determine which irq server to use, we check if
default_server, which is the id of the bootcpu, is still online.  But
default_server is a hardware cpu, not the logical cpu id needed to index
cpu_online_map.  

Since the default server can only go offline during a cpu hotplug event,
explicitly check the default server and choose the new one when we move
irqs away from the cpu being offlined.

This has the added benefit of only needing the boot_cpuid to be updated
and not relying on the cpu being marked offline during migrate_irqs_away.

Also, since xics_update_irq_servers only reads device tree information, we
can call it before xics_init_host in xics_init_IRQ and then default_server
will always be valid when we can reach get_irq_server via the host ops.

Signed-off-by: Milton Miller <miltonm@bga.com>
---
I realized the hard vs soft cpu id as I was writing the patch description.
get_irq_server is called from unmask and set affinity.  

The effects of this bug would be the increased pathlength to walk the
device tree in the unmask and set affinity methods, and possible failure
to migrate irqs during cpu hotplug removal (not likely to be seen on
until a sequence of remove and adds is performed).

This check dates to 2.6.25 via de0723dcca6e593a12a259798a54eb0e82628fb8.

Index: next.git/arch/powerpc/platforms/pseries/xics.c
===================================================================
--- next.git.orig/arch/powerpc/platforms/pseries/xics.c	2008-10-04 16:36:07.000000000 -0500
+++ next.git/arch/powerpc/platforms/pseries/xics.c	2008-10-04 16:36:09.000000000 -0500
@@ -208,9 +208,6 @@ static int get_irq_server(unsigned int v
 	cpumask_t cpumask = irq_desc[virq].affinity;
 	cpumask_t tmp = CPU_MASK_NONE;
 
-	if (! cpu_isset(default_server, cpu_online_map))
-		xics_update_irq_servers();
-
 	if (!distribute_irqs)
 		return default_server;
 
@@ -685,8 +682,8 @@ void __init xics_init_IRQ(void)
 	if (found == 0)
 		return;
 
-	xics_init_host();
 	xics_update_irq_servers();
+	xics_init_host();
 
 	if (firmware_has_feature(FW_FEATURE_LPAR))
 		ppc_md.get_irq = xics_get_irq_lpar;
@@ -779,6 +776,10 @@ void xics_migrate_irqs_away(void)
 	int cpu = smp_processor_id(), hw_cpu = hard_smp_processor_id();
 	unsigned int irq, virq;
 
+	/* If we used to be the default server, move to the new "boot_cpuid" */
+	if (hw_cpu == default_server)
+		xics_update_irq_servers();
+
 	/* Reject any interrupt that was queued to us... */
 	xics_set_cpu_priority(0);