Comment 3 for bug 1916879

From the linked Octavia bug https://storyboard.openstack.org/#!/story/2008651 I see that nova got error from libvirt.

2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [req-a71cf7b7-8198-42d4-99b3-e23951e17dd3 28a680c9c5ca4c05a22a29afed378311 6a6da86833a14179b295badec574d4c6 - default default] [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] attaching network adapter failed.: libvirt.libvirtError: Unable to read from monitor: Connection reset by peer
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] Traceback (most recent call last):
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] File "/usr/lib/python3.6/site-packages/nova/virt/libvirt/driver.py", line 2200, in attach_interface
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] guest.attach_device(cfg, persistent=True, live=live)
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] File "/usr/lib/python3.6/site-packages/nova/virt/libvirt/guest.py", line 293, in attach_device
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] self._domain.attachDeviceFlags(device_xml, flags=flags)
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] File "/usr/lib/python3.6/site-packages/eventlet/tpool.py", line 190, in doit
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] result = proxy_call(self._autowrap, f, *args, **kwargs)
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] File "/usr/lib/python3.6/site-packages/eventlet/tpool.py", line 148, in proxy_call
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] rv = execute(f, *args, **kwargs)
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] File "/usr/lib/python3.6/site-packages/eventlet/tpool.py", line 129, in execute
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] six.reraise(c, e, tb)
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] File "/usr/lib/python3.6/site-packages/six.py", line 703, in reraise
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] raise value
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] File "/usr/lib/python3.6/site-packages/eventlet/tpool.py", line 83, in tworker
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] rv = meth(*args, **kwargs)
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] File "/usr/lib64/python3.6/site-packages/libvirt.py", line 630, in attachDeviceFlags
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] if ret == -1: raise libvirtError ('virDomainAttachDeviceFlags() failed', dom=self)
2021-02-23 14:22:50.148 8 ERROR nova.virt.libvirt.driver [instance: 7385598b-6161-4012-86b1-6d5a88a6e85c] libvirt.libvirtError: Unable to read from monitor: Connection reset by peer

So I suggest to report this error to the libvirt community, https://libvirt.org/bugs.html with the above logs, and the libvirt / qemu logs attached.