[PATCH] qemu: Properly handle setting of <iotune> for empty cdrom

[Peter Krempa]

When starting a VM with an empty cdrom which has <iotune> configured the
startup fails as qemu is not happy about setting tuning for an empty
drive:

 error: internal error: unable to execute 'block_set_io_throttle', unexpected error: 'Device has no medium'

Resolve this by skipping the setting of throttling for empty drives and
updating the throttling when new medium is inserted into the drive.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/111
Signed-off-by: Peter Krempa <pkrempa at redhat.com>
---
 src/qemu/qemu_hotplug.c | 10 ++++++++++
 src/qemu/qemu_process.c |  4 ++++
 2 files changed, 14 insertions(+)

diff --git a/src/qemu/qemu_hotplug.c b/src/qemu/qemu_hotplug.c
index 9b93f256e8..57635cd419 100644
--- a/src/qemu/qemu_hotplug.c
+++ b/src/qemu/qemu_hotplug.c
@@ -558,6 +558,16 @@ qemuDomainChangeMediaBlockdev(virQEMUDriverPtr driver,
                                                  nodename);
     }

+    /* set throttling for the new image */
+    if (rc == 0 &&
+        !virStorageSourceIsEmpty(newsrc) &&
+        qemuDiskConfigBlkdeviotuneEnabled(disk)) {
+        rc = qemuMonitorSetBlockIoThrottle(priv->mon, NULL,
+                                           diskPriv->qomName,
+                                           &disk->blkdeviotune,
+                                           true, true, true);
+    }
+
     if (rc == 0)
         rc = qemuMonitorBlockdevTrayClose(priv->mon, diskPriv->qomName);

diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index e7421b415f..414e9327d2 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -6838,6 +6838,10 @@ qemuProcessSetupDiskThrottlingBlockdev(virQEMUDriverPtr driver,
         if (qemuDiskBusIsSD(disk->bus))
             continue;

+        /* Setting throttling for empty drives fails */
+        if (virStorageSourceIsEmpty(disk->src))
+            continue;
+
         if (!qemuDiskConfigBlkdeviotuneEnabled(disk))
             continue;

-- 
2.29.2