x86: don't change affinity with interrupt unmasked

With ->startup unmasking the IRQ, setting the affinity afterwards
without masking the IRQ again is invalid namely for MSI (address and
data can't be updated atomically and may - at least for MSI-X - be
cached while unmasked).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/irq.c

@@ -1608,12 +1608,13 @@ int pirq_guest_bind(struct vcpu *v, struct pirq *pirq, int will_share)
         init_timer(&action->eoi_timer, irq_guest_eoi_timer_fn, desc, 0);
 
         desc->status |= IRQ_GUEST;
-        desc->status &= ~IRQ_DISABLED;
-        desc->handler->startup(desc);
 
         /* Attempt to bind the interrupt target to the correct CPU. */
         if ( !opt_noirqbalance && (desc->handler->set_affinity != NULL) )
             desc->handler->set_affinity(desc, cpumask_of(v->processor));
+
+        desc->status &= ~IRQ_DISABLED;
+        desc->handler->startup(desc);
     }
     else if ( !will_share || !action->shareable )
     {