--- a/drivers/pci/msi/msi.c
+++ b/drivers/pci/msi/msi.c
@@ -569,34 +569,56 @@ static void __iomem *msix_map_region(str
 	return ioremap(phys_addr, nr_entries * PCI_MSIX_ENTRY_SIZE);
 }
 
-static int msix_setup_msi_descs(struct pci_dev *dev, void __iomem *base,
-				struct msix_entry *entries, int nvec,
-				struct irq_affinity_desc *masks)
+/**
+ * msix_prepare_msi_desc - Prepare a half initialized MSI descriptor for operation
+ * @dev:	The PCI device for which the descriptor is prepared
+ * @desc:	The MSI descriptor for preparation
+ *
+ * This is separate from msix_setup_msi_descs() below to handle dynamic
+ * allocations for MSI-X after initial enablement.
+ *
+ * Ideally the whole MSI-X setup would work that way, but there is no way to
+ * support this for the legacy arch_setup_msi_irqs() mechanism and for the
+ * fake irq domains like the x86 XEN one. Sigh...
+ *
+ * The descriptor is zeroed and only @desc::msi_index and @desc::affinity
+ * are set. When called from msix_setup_msi_descs() then the is_virtual
+ * attribute is initialized as well.
+ *
+ * Fill in the rest.
+ */
+void msix_prepare_msi_desc(struct pci_dev *dev, struct msi_desc *desc)
+{
+	desc->nvec_used				= 1;
+	desc->pci.msi_attrib.is_msix		= 1;
+	desc->pci.msi_attrib.is_64		= 1;
+	desc->pci.msi_attrib.default_irq	= dev->irq;
+	desc->pci.mask_base			= dev->msix_base;
+	desc->pci.msi_attrib.can_mask		= !pci_msi_ignore_mask &&
+						  !desc->pci.msi_attrib.is_virtual;
+
+	if (desc->pci.msi_attrib.can_mask) {
+		void __iomem *addr = pci_msix_desc_addr(desc);
+
+		desc->pci.msix_ctrl = readl(addr + PCI_MSIX_ENTRY_VECTOR_CTRL);
+	}
+}
+
+static int msix_setup_msi_descs(struct pci_dev *dev, struct msix_entry *entries,
+				int nvec, struct irq_affinity_desc *masks)
 {
 	int ret = 0, i, vec_count = pci_msix_vec_count(dev);
 	struct irq_affinity_desc *curmsk;
 	struct msi_desc desc;
-	void __iomem *addr;
 
 	memset(&desc, 0, sizeof(desc));
 
-	desc.nvec_used			= 1;
-	desc.pci.msi_attrib.is_msix	= 1;
-	desc.pci.msi_attrib.is_64	= 1;
-	desc.pci.msi_attrib.default_irq	= dev->irq;
-	desc.pci.mask_base		= base;
-
 	for (i = 0, curmsk = masks; i < nvec; i++, curmsk++) {
 		desc.msi_index = entries ? entries[i].entry : i;
 		desc.affinity = masks ? curmsk : NULL;
 		desc.pci.msi_attrib.is_virtual = desc.msi_index >= vec_count;
-		desc.pci.msi_attrib.can_mask = !pci_msi_ignore_mask &&
-					       !desc.pci.msi_attrib.is_virtual;
 
-		if (desc.pci.msi_attrib.can_mask) {
-			addr = pci_msix_desc_addr(&desc);
-			desc.pci.msix_ctrl = readl(addr + PCI_MSIX_ENTRY_VECTOR_CTRL);
-		}
+		msix_prepare_msi_desc(dev, &desc);
 
 		ret = msi_insert_msi_desc(&dev->dev, &desc);
 		if (ret)
@@ -629,9 +651,8 @@ static void msix_mask_all(void __iomem *
 		writel(ctrl, base + PCI_MSIX_ENTRY_VECTOR_CTRL);
 }
 
-static int msix_setup_interrupts(struct pci_dev *dev, void __iomem *base,
-				 struct msix_entry *entries, int nvec,
-				 struct irq_affinity *affd)
+static int msix_setup_interrupts(struct pci_dev *dev, struct msix_entry *entries,
+				 int nvec, struct irq_affinity *affd)
 {
 	struct irq_affinity_desc *masks = NULL;
 	int ret;
@@ -640,7 +661,7 @@ static int msix_setup_interrupts(struct
 		masks = irq_create_affinity_masks(nvec, affd);
 
 	msi_lock_descs(&dev->dev);
-	ret = msix_setup_msi_descs(dev, base, entries, nvec, masks);
+	ret = msix_setup_msi_descs(dev, entries, nvec, masks);
 	if (ret)
 		goto out_free;
 
@@ -678,7 +699,6 @@ static int msix_setup_interrupts(struct
 static int msix_capability_init(struct pci_dev *dev, struct msix_entry *entries,
 				int nvec, struct irq_affinity *affd)
 {
-	void __iomem *base;
 	int ret, tsize;
 	u16 control;
 
@@ -696,15 +716,13 @@ static int msix_capability_init(struct p
 	pci_read_config_word(dev, dev->msix_cap + PCI_MSIX_FLAGS, &control);
 	/* Request & Map MSI-X table region */
 	tsize = msix_table_size(control);
-	base = msix_map_region(dev, tsize);
-	if (!base) {
+	dev->msix_base = msix_map_region(dev, tsize);
+	if (!dev->msix_base) {
 		ret = -ENOMEM;
 		goto out_disable;
 	}
 
-	dev->msix_base = base;
-
-	ret = msix_setup_interrupts(dev, base, entries, nvec, affd);
+	ret = msix_setup_interrupts(dev, entries, nvec, affd);
 	if (ret)
 		goto out_disable;
 
@@ -719,7 +737,7 @@ static int msix_capability_init(struct p
 	 * which takes the MSI-X mask bits into account even
 	 * when MSI-X is disabled, which prevents MSI delivery.
 	 */
-	msix_mask_all(base, tsize);
+	msix_mask_all(dev->msix_base, tsize);
 	pci_msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_MASKALL, 0);
 
 	pcibios_free_irq(dev);
--- a/drivers/pci/msi/msi.h
+++ b/drivers/pci/msi/msi.h
@@ -84,6 +84,8 @@ static inline __attribute_const__ u32 ms
 	return (1 << (1 << desc->pci.msi_attrib.multi_cap)) - 1;
 }
 
+void msix_prepare_msi_desc(struct pci_dev *dev, struct msi_desc *desc);
+
 /* Subsystem variables */
 extern int pci_msi_enable;
 
