[PATCH for-4.19] xen/x86: 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.

Note that the shuffling done by fixup_irqs() is racy: the old destination
target is not allowed to drain any pending interrupts before the new
destination is set, which can lead to spurious 'No irq handler for vector ...'
messages.  While the proposed change doesn't fix the issue, limiting the
amount of shuffling to only strictly necessary reduces the chances of stray
interrupts.

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

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 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 413994d2133b..33dd7667137b 100644
--- a/xen/arch/x86/include/asm/irq.h
+++ b/xen/arch/x86/include/asm/irq.h
@@ -130,7 +130,7 @@ 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 3b951d81bd6d..223f813f6ceb 100644
--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -2527,7 +2527,7 @@ 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;
@@ -2576,7 +2576,12 @@ void fixup_irqs(const cpumask_t *mask, bool verbose)
                 release_old_vec(desc);
         }
 
-        if ( !desc->action || cpumask_subset(desc->affinity, mask) )
+        /*
+         * Avoid shuffling the interrupt around if it's assigned to a CPU set
+         * that's all covered by the requested affinity mask.
+         */
+        cpumask_and(affinity, desc->arch.cpu_mask, &cpu_online_map);
+        if ( !desc->action || cpumask_subset(affinity, mask) )
         {
             spin_unlock(&desc->lock);
             continue;
-- 
2.44.0