intel-iommu: Accept 64-bit writes to FEADDR

Xen is doing this [1] and currently triggers an abort.

[1] http://xenbits.xenproject.org/gitweb/?p=xen.git;a=blob;f=xen/drivers/passthrough/vtd/iommu.c;h=daaed0abbdd06b6ba3d948ea103aadf02651e83c;hb=refs/heads/master#l1108Reported-by: NLuis Lloret <luis_lloret@mentor.com>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: NPeter Xu <peterx@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/i386/intel_iommu.c

@@ -2129,8 +2129,15 @@ static void vtd_mem_write(void *opaque, hwaddr addr,
 
     /* Fault Event Address Register, 32-bit */
     case DMAR_FEADDR_REG:
-        assert(size == 4);
-        vtd_set_long(s, addr, val);
+        if (size == 4) {
+            vtd_set_long(s, addr, val);
+        } else {
+            /*
+             * While the register is 32-bit only, some guests (Xen...) write to
+             * it with 64-bit.
+             */
+            vtd_set_quad(s, addr, val);
+        }
         break;
 
     /* Fault Event Upper Address Register, 32-bit */