Add a reverse-mapping function to return the interrupt vector for
any CPU if interrupt affinity is enabled.

Signed-off-by: Hannes Reinecke <hare@suse.com>
---
 drivers/pci/msi.c   | 36 ++++++++++++++++++++++++++++++++++++
 include/linux/pci.h |  1 +
 2 files changed, 37 insertions(+)

diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
index bfdd074..de5ed32 100644
--- a/drivers/pci/msi.c
+++ b/drivers/pci/msi.c

@@ -1302,6 +1302,42 @@ const struct cpumask *pci_irq_get_affinity(struct pci_dev *dev, int nr)
 }
 EXPORT_SYMBOL(pci_irq_get_affinity);
 
+/**
+ * pci_irq_get_affinity_vector - return the vector number for a given CPU
+ * @dev:	PCI device to operate on
+ * @cpu:	cpu number
+ *
+ * Returns the vector number for CPU @cpu or a negative error number
+ * if interrupt affinity is not set.
+ */
+int pci_irq_get_affinity_vector(struct pci_dev *dev, int cpu)
+{
+	if (dev->msix_enabled) {
+		struct msi_desc *entry;
+
+		for_each_pci_msi_entry(entry, dev) {
+			if (cpumask_test_cpu(cpu, entry->affinity))