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Draft: core,libnm: don't touch device TC configuration by default

Beniamino Galvani requested to merge bg/tc-ignore into main

NetworkManager supports a very limited set of qdiscs. If users want to configure a unsupported qdisc, they need to do it outside of NetworkManager using tc.

The problem is that NM also removes all qdiscs and filters during activation if the connection doesn't contain a TC setting. Therefore, setting TC configuration outside of NM is hard because users need to do it after the connection is up (for example through a dispatcher script).

Let NM consider the presence (or absence) of a TC setting in the connection to determine whether NM should configure (or not) qdiscs and filters on the interface. We already do something similar for SR-IOV configuration.

Since new connections don't have the TC setting, the new behavior (ignore existing configuration) will be the default. The impact of this change in different scenarios is:

  • the user previously configured TC settings via NM. This continues to work as before;

  • the user didn't care about qdiscs and filters; NM removed all qdiscs upon activation, and so the default qdisc from kernel was used. After this change, NM will not touch qdiscs and the default qdisc will be used, as before;

  • the user set a different qdisc via tc and NM cleared it during activation. Now this will work as expected.

So, the new default behavior seems better than the previous one.

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