virtio: improve virtqueue mapping error messages

Improve error reporting when virtqueue ring mapping fails by including a
device identifier in the error message.

Introduce a helper qdev_get_printable_name() in qdev-core, which returns
either:

 - the device ID, if explicitly provided (e.g. -device ...,id=foo)
 - the QOM path from qdev_get_dev_path(dev) otherwise
 - "<unknown device>" as a fallback when no identifier is present

This makes it easier to identify which device triggered the error in
multi-device setups or when debugging complex guest configurations.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/230
Buglink: https://bugs.launchpad.net/qemu/+bug/1919021
Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Alessandro Ratti <alessandro@0x65c.net>
Message-Id: <20250924093138.559872-2-alessandro@0x65c.net>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 29 insertions, 0 deletions

diff --git a/hw/core/qdev.c b/hw/core/qdev.c
index f600226176..fab42a7270 100644
--- a/hw/core/qdev.c
+++ b/hw/core/qdev.c
@@ -411,6 +411,35 @@ char *qdev_get_dev_path(DeviceState *dev)
     return NULL;
 }
 
+const char *qdev_get_printable_name(DeviceState *vdev)
+{
+    /*
+     * Return device ID if explicity set
+     * (e.g. -device virtio-blk-pci,id=foo)
+     * This allows users to correlate errors with their custom device
+     * names.
+     */
+    if (vdev->id) {
+        return vdev->id;
+    }
+    /*
+     * Fall back to the canonical QOM device path (eg. ID for PCI
+     * devices).
+     * This ensures the device is still uniquely and meaningfully
+     * identified.
+     */
+    const char *path = qdev_get_dev_path(vdev);
+    if (path) {
+        return path;
+    }
+
+    /*
+     * Final fallback: if all else fails, return a placeholder string.
+     * This ensures the error message always contains a valid string.
+     */
+    return "<unknown device>";
+}
+
 void qdev_add_unplug_blocker(DeviceState *dev, Error *reason)
 {
     dev->unplug_blockers = g_slist_prepend(dev->unplug_blockers, reason);