netdev - Re: [PATCH rdma-next 1/1] RDMA/mana_ib: Set correct device into ib

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Date: Wed, 26 Jun 2024 18:19:41 +0000
From: Konstantin Taranov <kotaranov@...rosoft.com>
To: Stephen Hemminger <stephen@...workplumber.org>, Leon Romanovsky
	<leon@...nel.org>, Haiyang Zhang <haiyangz@...rosoft.com>
CC: Konstantin Taranov <kotaranov@...ux.microsoft.com>, Wei Hu
	<weh@...rosoft.com>, "sharmaajay@...rosoft.com" <sharmaajay@...rosoft.com>,
	Long Li <longli@...rosoft.com>, "jgg@...pe.ca" <jgg@...pe.ca>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>, linux-netdev
	<netdev@...r.kernel.org>
Subject: Re: [PATCH rdma-next 1/1] RDMA/mana_ib: Set correct device into ib

> > > > On Wed, Jun 26, 2024 at 09:05:05AM +0000, Konstantin Taranov wrote:
> > > > > > > When mc->ports[0] is not slave, use it in the set_netdev.
> > > > > > > When mana is used in netvsc, the stored net devices in mana
> > > > > > > are slaves and GIDs should be taken from their master devices.
> > > > > > > In the baremetal case, the mc->ports devices will not be slaves.
> > > > > >
> > > > > > I wonder, why do you have "... | IFF_SLAVE" in
> > > > > > __netvsc_vf_setup() in a first place? Isn't IFF_SLAVE is supposed to
> be set by bond driver?
> > > > > >
> > > > >
> > > > > I guess it is just a valid use of the IFF_SLAVE bit. In the bond
> > > > > case it is also set as a BOND netdev. The IFF_SLAVE helps to show
> users that another master
> > > > > netdev should be used for networking. But I am not an expert in
> netvsc.
> > > >
> > > > The thing is that netvsc is virtual device like many others, but
> > > > it is the only one who uses IFF_SLAVE bit. The comment around that
> > > > bit says "slave of a load balancer.", which is not the case
> > > > according to the Hyper-V documentation.
> > > >
> > > > You will need to get Ack from netdev maintainers to rely on
> > > > IFF_SLAVE bit in the way you are relying on it now.
> > >
> > > This is used to tell userspace tools to not interact directly with the device.
> > > For example, it is used when VF is connected to netvsc device.
> > > It prevents things like IPv6 local address, and Network Manager won't
> modify device.
> >
> > You described how hyper-v uses it, but I'm interested to get
> > acknowledgment that it is a valid use case for IFF_SLAVE, despite sentence
> written in the comment.
> 
> There is no documented semantics around any of the IF flags, only historical
> precedent used by bond, team and bridge drivers. Initially Hyper-V VF used
> bonding but it was impossibly difficult to make this work across all versions of
> Linux, so transparent VF support was added instead. Ideally, the VF device
> could be hidden from userspace but that required more kernel modifications
> than would be accepted.

Thanks Stephen for the explanation!

I am also CCing Haiyang, who maintains Hyper-V netvsc.