netdev - Re: [PATCH] net/ethernet : set default assignment identifier to NET_NAME

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Date:   Sat, 2 Apr 2022 11:16:12 +1100
From:   Ian Wienand <iwienand@...hat.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] net/ethernet : set default assignment identifier to
 NET_NAME_ENUM

Thanks for review

On Fri, Apr 01, 2022 at 03:13:27PM +0200, Andrew Lunn wrote:
> On Fri, Apr 01, 2022 at 05:34:30PM +1100, Ian Wienand wrote:
> > As noted in the original commit 685343fc3ba6 ("net: add
> > name_assign_type netdev attribute")
> > 
> >   ... when the kernel has given the interface a name using global
> >   device enumeration based on order of discovery (ethX, wlanY, etc)
> >   ... are labelled NET_NAME_ENUM.
> > 
> > That describes this case, so set the default for the devices here to
> > NET_NAME_ENUM to better help userspace tools to know if they might
> > like to rename them.

> Is this potentially an ABI change

I don't think this counts as an ABI change; it's a fixed-size flag and
designed to be updated with more specific information on a
case-by-case basis?

> and we will get surprises when
> interfaces which were not previously renamed now are? It would be nice
> to see some justification this is actually safe to do.

I came to this through inconsistency of behaviour in a heterogeneous
OpenStack cloud environment.  Most of the clouds are kvm-based and
have NICS using virtio which fills in
/sys/class/net/<interface>/name_assign_type and this bubbles up
through udev/systemd/general magic to get the devices renamed.
However we have one outlier using Xen and the xen-netfront driver,
which I traced to here, where name_assign_type isn't set.  So I think
it's a consistency win to have it reporting itself as NET_NAME_ENUM
here.

Thanks,

-i