x86/irq: limit interrupt movement done by fixup_irqs()

The current check used in fixup_irqs() to decide whether to move around
interrupts is based on the affinity mask, but such mask can have all bits set,
and hence is unlikely to be a subset of the input mask.  For example if an
interrupt has an affinity mask of all 1s, any input to fixup_irqs() that's not
an all set CPU mask would cause that interrupt to be shuffled around
unconditionally.

What fixup_irqs() care about is evacuating interrupts from CPUs not set on the
input CPU mask, and for that purpose it should check whether the interrupt is
assigned to a CPU not present in the input mask.  Assume that ->arch.cpu_mask
is a subset of the ->affinity mask, and keep the current logic that resets the
->affinity mask if the interrupt has to be shuffled around.

Doing the affinity movement based on ->arch.cpu_mask requires removing the
special handling to ->arch.cpu_mask done for high priority vectors, otherwise
the adjustment done to cpu_mask makes them always skip the CPU interrupt
movement.

While there also adjust the comment as to the purpose of fixup_irqs().

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-By: Oleksii Kurochko <oleksii.kurochko@gmail.com>

xen/arch/x86/include/asm/irq.h

@@ -168,7 +168,7 @@ void free_domain_pirqs(struct domain *d);
 int map_domain_emuirq_pirq(struct domain *d, int pirq, int emuirq);
 int unmap_domain_pirq_emuirq(struct domain *d, int pirq);
 
-/* Reset irq affinities to match the given CPU mask. */
+/* Evacuate interrupts assigned to CPUs not present in the input CPU mask. */
 void fixup_irqs(const cpumask_t *mask, bool verbose);
 void fixup_eoi(void);
 

xen/arch/x86/irq.c

@@ -2516,7 +2516,7 @@ static int __init cf_check setup_dump_irqs(void)
 }
 __initcall(setup_dump_irqs);
 
-/* Reset irq affinities to match the given CPU mask. */
+/* Evacuate interrupts assigned to CPUs not present in the input CPU mask. */
 void fixup_irqs(const cpumask_t *mask, bool verbose)
 {
     unsigned int irq;
@@ -2540,19 +2540,15 @@ void fixup_irqs(const cpumask_t *mask, bool verbose)
 
         vector = irq_to_vector(irq);
         if ( vector >= FIRST_HIPRIORITY_VECTOR &&
-             vector <= LAST_HIPRIORITY_VECTOR )
+             vector <= LAST_HIPRIORITY_VECTOR &&
+             desc->handler == &no_irq_type )
         {
-            cpumask_and(desc->arch.cpu_mask, desc->arch.cpu_mask, mask);
-
             /*
              * This can in particular happen when parking secondary threads
              * during boot and when the serial console wants to use a PCI IRQ.
              */
-            if ( desc->handler == &no_irq_type )
-            {
-                spin_unlock(&desc->lock);
-                continue;
-            }
+            spin_unlock(&desc->lock);
+            continue;
         }
 
         if ( desc->arch.move_cleanup_count )
@@ -2573,7 +2569,12 @@ void fixup_irqs(const cpumask_t *mask, bool verbose)
                                affinity);
         }
 
-        if ( !desc->action || cpumask_subset(desc->affinity, mask) )
+        /*
+         * Avoid shuffling the interrupt around as long as current target CPUs
+         * are a subset of the input mask.  What fixup_irqs() cares about is
+         * evacuating interrupts from CPUs not in the input mask.
+         */
+        if ( !desc->action || cpumask_subset(desc->arch.cpu_mask, mask) )
         {
             spin_unlock(&desc->lock);
             continue;