Index: head/sys/amd64/include/intr_machdep.h
===================================================================
--- head/sys/amd64/include/intr_machdep.h
+++ head/sys/amd64/include/intr_machdep.h
@@ -149,6 +149,8 @@
 extern struct mtx icu_lock;
 extern int elcr_found;
 
+extern int msix_disable_migration;
+
 #ifndef DEV_ATPIC
 void	atpic_reset(void);
 #endif
Index: head/sys/i386/include/intr_machdep.h
===================================================================
--- head/sys/i386/include/intr_machdep.h
+++ head/sys/i386/include/intr_machdep.h
@@ -140,6 +140,8 @@
 extern struct mtx icu_lock;
 extern int elcr_found;
 
+extern int msix_disable_migration;
+
 #ifndef DEV_ATPIC
 void	atpic_reset(void);
 #endif
Index: head/sys/x86/x86/msi.c
===================================================================
--- head/sys/x86/x86/msi.c
+++ head/sys/x86/x86/msi.c
@@ -149,12 +149,16 @@
 	.pic_reprogram_pin = NULL,
 };
 
-/*
+/**
  * Xen hypervisors prior to 4.6.0 do not properly handle updates to
  * enabled MSI-X table entries.  Allow migration of MSI-X interrupts
- * to be disabled via a tunable.
+ * to be disabled via a tunable. Values have the following meaning:
+ *
+ * -1: automatic detection by FreeBSD
+ *  0: enable migration
+ *  1: disable migration
  */
-static int msix_disable_migration = 0;
+int msix_disable_migration = -1;
 SYSCTL_INT(_machdep, OID_AUTO, disable_msix_migration, CTLFLAG_RDTUN,
     &msix_disable_migration, 0,
     "Disable migration of MSI-X interrupts between CPUs");
@@ -312,6 +316,11 @@
 		return;
 	}
 
+	if (msix_disable_migration == -1) {
+		/* The default is to allow migration of MSI-X interrupts. */
+		msix_disable_migration = 0;
+	}
+
 	msi_enabled = 1;
 	intr_register_pic(&msi_pic);
 	mtx_init(&msi_lock, "msi", NULL, MTX_DEF);
Index: head/sys/x86/xen/hvm.c
===================================================================
--- head/sys/x86/xen/hvm.c
+++ head/sys/x86/xen/hvm.c
@@ -134,9 +134,29 @@
 		return (ENXIO);
 
 	if (init_type == XEN_HVM_INIT_COLD) {
+		int major, minor;
+
 		do_cpuid(base + 1, regs);
-		printf("XEN: Hypervisor version %d.%d detected.\n",
-		    regs[0] >> 16, regs[0] & 0xffff);
+
+		major = regs[0] >> 16;
+		minor = regs[0] & 0xffff;
+		printf("XEN: Hypervisor version %d.%d detected.\n", major,
+			minor);
+
+		if (((major < 4) || (major == 4 && minor <= 5)) &&
+		    msix_disable_migration == -1) {
+			/*
+			 * Xen hypervisors prior to 4.6.0 do not properly
+			 * handle updates to enabled MSI-X table entries,
+			 * so disable MSI-X interrupt migration in that
+			 * case.
+			 */
+			if (bootverbose)
+				printf(
+"Disabling MSI-X interrupt migration due to Xen hypervisor bug.\n"
+"Set machdep.msix_disable_migration=0 to forcefully enable it.\n");
+			msix_disable_migration = 1;
+		}
 	}
 
 	/*