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Description of problem:
Error message weird when there is no enough pci slot for hotplug interface

Version-Release number of selected component (if applicable):
libvirt-4.5.0-1.el7.x86_64


How reproducible:
100%

Steps to Reproduce:
1. start a guest, then hotplug interface with below command
# for i in `seq 32`;do  echo -----$i------; virsh attach-interface rhel network default; done
-----1------
Interface attached successfully
......
-----21------
error: Failed to attach interface
error: Network filter binding not found: no nwfilter binding for port dev 'vnet20'
-----22------
error: Failed to attach interface
error: Network filter binding not found: no nwfilter binding for port dev 'vnet20'


Actual results:
When there is no enough pci slots, the error messages is weird about network filter

Expected results:
the error message should be like:
error: Failed to attach interface
error: internal error: No more available PCI slots
(test on libvirt-4.4.0-2.el7.x86_64)

Additional info:

It sounds like the nwfilter "error" is happening during cleanup (we're attempting to removing nwfilter bindings that we hadn't yet created), so it's obscuring the original error. I don't have time to look in more detail now.

https://www.redhat.com/archives/libvir-list/2018-July/msg00882.html

Fixed with v4.5.0-147-gf7b55665cc:

commit f7b55665cc6b805bc4c34e1ec5908df31084ef5c
Author: Katerina Koukiou <kkoukiou>
Date:   Fri Jul 13 17:01:44 2018 +0200

    qemu: hotplug: don't overwrite error message in qemuDomainAttachNetDevice

Test on libvirt-4.5.0-4.el7.x86_64 with the same steps in comment 0, the result is as expected, set this bug to be verified.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2018:3113