[PATCH for-4.19 5/9] x86/irq: limit interrupt movement done by fixup_irqs()

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

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.

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

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Do not AND ->arch.cpu_mask with cpu_online_map.
 - Keep using cpumask_subset().
 - Integrate into bigger series.
---
 xen/arch/x86/include/asm/irq.h | 2 +-
 xen/arch/x86/irq.c             | 9 +++++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/include/asm/irq.h b/xen/arch/x86/include/asm/irq.h
index 80a3aa7a88b9..b7dc75d0acbd 100644
--- a/xen/arch/x86/include/asm/irq.h
+++ b/xen/arch/x86/include/asm/irq.h
@@ -156,7 +156,7 @@ 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);
 
diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
index 9716e00e873b..1b7127090377 100644
--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -2525,7 +2525,7 @@ 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;
@@ -2582,7 +2582,12 @@ void fixup_irqs(const cpumask_t *mask, bool verbose)
                             &cpu_online_map);
         }
 
-        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;
-- 
2.44.0