Index: head/sys/x86/x86/msi.c
===================================================================
--- head/sys/x86/x86/msi.c
+++ head/sys/x86/x86/msi.c
@@ -46,6 +46,7 @@
 #include <sys/malloc.h>
 #include <sys/mutex.h>
 #include <sys/sx.h>
+#include <sys/sysctl.h>
 #include <sys/systm.h>
 #include <x86/apicreg.h>
 #include <machine/cputypes.h>
@@ -148,6 +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.
+ */
+static int msix_disable_migration = 0;
+SYSCTL_INT(_machdep, OID_AUTO, disable_msix_migration, CTLFLAG_RDTUN,
+    &msix_disable_migration, 0,
+    "Disable migration of MSI-X interrupts between CPUs");
+
 static int msi_enabled;
 static int msi_last_irq;
 static struct mtx msi_lock;
@@ -226,6 +237,9 @@
 	if (msi->msi_first != msi)
 		return (EINVAL);
 
+	if (msix_disable_migration && msi->msi_msix)
+		return (EINVAL);
+
 	/* Store information to free existing irq. */
 	old_vector = msi->msi_vector;
 	old_id = msi->msi_cpu;