[RFC] Faster libvirtd restart with nwfilter rules, one more time

Hi, all. Some time ago I posted RFC [1] concerning an issue of unresponsive libvird during restart if there is large number of VMs that have network filters on their interfaces. It was identified that in most cases we don't need actually to reinstall network filter rules on daemon restart. Thus I proposed patches [2] that check whether we need to reapply rules or not. The first version has a drawback that daemon won't reapply rules if someone mangled them between daemon stop and start (and this can be done just by restarting firewalld). The second one is just ugly :) Around that time Florian Westphal in a letter off the mailing list suggested to use {iptables|ebtables}-restore to apply rules in one binary call. These binaries has --noflush option so that we won't reset current state of tables. We also need one more -L call for iptables/ebtables to query current filter state to be able to construct input for restore binaries. I wonder can we use this approach? I see currently only one issue - we won't use firealld to spawn rules. But why we need to spawn rules through firewalld if it present? We use passthrough mode anyway. I tried to dig history for hints but didn't found anything. Patch [3] introduced spawning rules thru firewalld-cmd. Nikolay [1] [RFC] Faster libvirtd restart with nwfilter rules https://www.redhat.com/archives/libvir-list/2018-September/msg01206.html [2] nwfilter: don't reinstantiate filters if they are not changed v1: https://www.redhat.com/archives/libvir-list/2018-October/msg00904.html v2: https://www.redhat.com/archives/libvir-list/2018-October/msg01317.html [3] network: use firewalld instead of iptables, when available v0: https://www.redhat.com/archives/libvir-list/2012-April/msg01236.html v1: https://www.redhat.com/archives/libvir-list/2012-August/msg00447.html ... v4: https://www.redhat.com/archives/libvir-list/2012-August/msg01097.html