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

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Date: Thu, 27 Jun 2024 10:44:02 +0000
From: Konstantin Taranov <kotaranov@...rosoft.com>
To: Leon Romanovsky <leon@...nel.org>, Haiyang Zhang <haiyangz@...rosoft.com>
CC: Stephen Hemminger <stephen@...workplumber.org>, 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

> > > > > > > > > > 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.
> > >
> >
> > Yes, netvsc sets the IFF_SLAVE on VF for the bonding.
> >
> > Acked-by: Haiyang Zhang <haiyangz@...rosoft.com>
> 
> Konstantin.
> 
> I don't want to be first and only one driver that uses this flag outside of
> netdev. So please add new function to netdev part of mana driver to return
> right ndev.
> 
> Something like that:
> struct net_device *mana__get_netdev(struct mana_context *mc) {
> 	struct net_device *ndev;
> 
> 	if (mana_ndev_is_slave(mc->ports[0]))
> 		ndev = netdev_master_upper_dev_get_rcu(mc->ports[0]);
> 	else
> 		ndev = mc->ports[0];
> 
> 	return ndev;
> }
> 
> And get Acks from netdev maintainers (Jakub, David, Eric, Paolo).

Ok. Makes sense.
I will just call it more exact:
mana_get_not_slave_netdev_rcu()