diff --git a/src/qemu/qemu_domain_address.c b/src/qemu/qemu_domain_address.c
index 2106b344eaa30e745a5ef5259fe9a28c7bcc508f..b5b863fe4d50bb5de352c4c0fa38e9e063f291a9 100644
--- a/src/qemu/qemu_domain_address.c
+++ b/src/qemu/qemu_domain_address.c
@@ -645,6 +645,11 @@ qemuDomainDeviceCalculatePCIConnectFlags(virDomainDeviceDefPtr dev,
             return pcieFlags;
         }
 
+        /* mdevs don't have corresponding files in /sys that we can poke to
+         * try and figure out whether they are legacy PCI or PCI Express, so
+         * the logic below would never work; instead, we just go ahead and
+         * assume they're PCI Express. This is a very reasonable assumption,
+         * as all current mdev-capable devices are indeed PCI Express */
         if (hostdev->source.subsys.type == VIR_DOMAIN_HOSTDEV_SUBSYS_TYPE_MDEV)
             return pcieFlags;