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Re: [libvirt] [PATCH v2 1/1] qemu_hotplug.c: check disk address before hotplug

On 1/25/19 8:27 PM, John Ferlan wrote:

On 1/18/19 1:59 PM, Daniel Henrique Barboza wrote:
In a case where we want to hotplug the following disk:

<disk type='file' device='disk'>
     <address type='drive' controller='0' bus='0' target='0' unit='0'/>

In a QEMU guest that has a single OS disk, as follows:

<disk type='file' device='disk'>
     <address type='drive' controller='0' bus='0' target='0' unit='0'/>

What happens is that the existing guest disk will receive the ID
'scsi0-0-0-0' due to how Libvirt calculate the alias based on
the address in qemu_alias.c, qemuAssignDeviceDiskAlias. When hotplugging
a disk that happens to have the same address, Libvirt will calculate
the same ID to it and attempt to device_add. QEMU will refuse it:

$ virsh attach-device dhb hp-disk-dup.xml
error: Failed to attach device from hp-disk-dup.xml
error: internal error: unable to execute QEMU command 'device_add': Duplicate ID 'scsi0-0-0-0' for device

And Libvirt follows it up with a cleanup code in qemuDomainAttachDiskGeneric
that ends up removing what supposedly is a faulty hotplugged disk but, in
this case, ends up being the original guest disk. This happens because Libvirt
doesn't differentiate the error received by QMP device_add.

An argument can be made for how QMP device_add should provide a different
error code for this scenario. A quicker way to solve the problem is
presented in this patch: let us check the address of disk to be attached and
see if there is already a disk with the same address in the VM definition.
In this case, error out without calling device_add.

After this patch, this is the result of the previous attach-device call:

$ ./run tools/virsh attach-device dhb ~/hp-disk-dup.xml
error: Failed to attach device from /home/danielhb/hp-disk-dup.xml
error: operation failed: attached disk address conflicts with existing disk 'scsi0-0-0-0'

Reported-by: Srikanth Aithal <bssrikanth in ibm com>
Signed-off-by: Daniel Henrique Barboza <danielhb413 gmail com>
  src/qemu/qemu_hotplug.c | 39 +++++++++++++++++++++++++++++++++++++++
  1 file changed, 39 insertions(+)

diff --git a/src/qemu/qemu_hotplug.c b/src/qemu/qemu_hotplug.c
index a1c3ca999b..4e6703f0b8 100644
--- a/src/qemu/qemu_hotplug.c
+++ b/src/qemu/qemu_hotplug.c
@@ -875,6 +875,36 @@ qemuDomainChangeEjectableMedia(virQEMUDriverPtr driver,
+ * qemuDomainFindDiskByAddress:
+ *
+ * Returns an existing disk in the VM definition that matches a given
+ * bus/controller/unit/target set, NULL in no match was found. */
+static virDomainDiskDefPtr
+qemuDomainFindDiskByAddress(virDomainDefPtr def,
+                            virDomainDeviceInfo info)
+    virDomainDeviceInfo vm_info;
Prefer to see something like devinfo or diskinfo - what you look at
isn't a vm (virtual machine)!


+    int idx;
+    for (idx = 0; idx < def->ndisks; idx++) {
+        vm_info = def->disks[idx]->info;> +        if ((vm_info.addr.drive.bus == info.addr.drive.bus) &&
+            (vm_info.addr.drive.controller == info.addr.drive.controller) &&
+            (vm_info.addr.drive.unit == info.addr.drive.unit)) {
This would seem to assuming something about the address type wouldn't
it?  A _virDomainDeviceInfo is structure that uses it's @type in order
to determine what type of address is being used...

Yeah, this function is being called under a conditional inside qemuDomainAttachDiskGeneric:

+    if ((disk->info.type == VIR_DOMAIN_DEVICE_ADDRESS_TYPE_DRIVE) &&
+        (vm_disk = qemuDomainFindDiskByAddress(vm->def, disk->info))) {

It would be clearer to simply move this conditional inside the function I guess.

Look at tests/qemuxml2xmloutdata/disk-virtio.xml and wonder what would

Suggestion - look for virDomainDeviceInfoAddressIsEqual and maybe even

Perhaps even search through domain_conf.c for 'ndisks' and see how other
iterations are done.

I'll take a look into those.

Just to make my intentions clear: in v1 I just made an alias check
after qemuAssingDeviceDiskAlias to see if the calculated alias already exists
in the domain def. With this new patch the idea was to check for the same
address instead - giving that the alias code uses the address to calculate, the
effect would be the same as v1 with a plus of avoiding a collision with user
created aliases. The logic of address checking was inspired by how the address
is used inside qemuAssignDeviceDiskAlias.



In a way I'm wondering how we got this far where XML with a duplicated
address was accepted. Of course I haven't thought/looked that hard at
the hotplug path lately either.


+                /* Address does not have target to compare. We have
+                 * a match. */
+                if (!info.addr.drive.target)
+                    return def->disks[idx];
+                else if (vm_info.addr.drive.target &&
+                         vm_info.addr.drive.target == info.addr.drive.target)
+                    return def->disks[idx];
+        }
+    }
+    return NULL;
   * qemuDomainAttachDiskGeneric:
@@ -888,12 +918,21 @@ qemuDomainAttachDiskGeneric(virQEMUDriverPtr driver,
      int ret = -1;
      qemuDomainObjPrivatePtr priv = vm->privateData;
      qemuHotplugDiskSourceDataPtr diskdata = NULL;
+    virDomainDiskDefPtr vm_disk = NULL;
      char *devstr = NULL;
      virQEMUDriverConfigPtr cfg = virQEMUDriverGetConfig(driver);
if (qemuHotplugPrepareDiskAccess(driver, vm, disk, NULL, false) < 0)
          goto cleanup;
+ if ((disk->info.type == VIR_DOMAIN_DEVICE_ADDRESS_TYPE_DRIVE) &&
+        (vm_disk = qemuDomainFindDiskByAddress(vm->def, disk->info))) {
+        virReportError(VIR_ERR_OPERATION_FAILED,
+                       _("attached disk address conflicts with existing "
+                         "disk '%s'"), vm_disk->info.alias);
+        goto error;
+    }
      if (qemuAssignDeviceDiskAlias(vm->def, disk, priv->qemuCaps) < 0)
          goto error;

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