Subject: Re: [PATCH] hv_netvsc: Set device flags for properly indicating bonding

From: Stephen Hemminger <[email-masked]>
Date: Fri, 13 Dec 2024 09:50:28 -0800

On Wed, 27 Nov 2024 11:42:50 -0800
[email-masked] wrote:

> hv_netvsc uses a subset of bonding features in that the master always
> has only one active slave. But it never properly setup those flags.
> 
> Other kernel APIs (e.g those in "include/linux/netdevice.h") check for
> IFF_MASTER, IFF_SLAVE and IFF_BONDING for determing if those are used
> in a master/slave setup. RDMA uses those APIs extensively when looking
> for master/slave devices.
> 
> Make hv_netvsc properly setup those flags.
> 
> Signed-off-by: Long Li <[email-masked]>

Linux networking has evolved a patch work of flags to network devices
but never really standardized on a way to express linked relationships
between network devices. There are several drivers do this in slighlty
different and overlapping ways: bonding, team, failover, hyperv, bridge
and others.

The current convention is to mark the primary device as IFF_MASTER
and each secondary device with IFF_SLAVE.  But not clear what the
right combination is if stacked more than one level.  Also, not clear
if userspace and other addressing should use or not use nested devices.

It would be ideal to deprecate and use different terms than
the non-inclusive terms master/slave
but probably that would have to have been done years ago (like 2.5).

For now, it makes sense for all the nested devices to use IFF_MASTER
and IFF_SLAVE consistently. It is not a good idea to set the priv_flag
for IFF_BONDING (or any of the other bits) which should be reserved
for one driver. 

If hyperv driver needs to it could add its own flag in netdev_priv_flags,
but it looks like that is running out of bits. May need to grow to 64 bits
or do some more work to add a new field for device type. I.e. there
are combinations of flags that are impossible and having one bit per
type leads to a problem. Fixing that is non trivial but not impossible
level of effort.

My thoughts, but I don't use or work on Hyper-v anymore.