linux-kernel - Re: [PATCH] vdpa/mlx5: Use random MAC for the vdpa net instance

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Date:   Mon, 30 Nov 2020 04:00:51 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Eli Cohen <elic@...dia.com>
Cc:     jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, lulu@...hat.com
Subject: Re: [PATCH] vdpa/mlx5: Use random MAC for the vdpa net instance

On Mon, Nov 30, 2020 at 08:27:46AM +0200, Eli Cohen wrote:
> On Sun, Nov 29, 2020 at 03:08:22PM -0500, Michael S. Tsirkin wrote:
> > On Sun, Nov 29, 2020 at 08:43:51AM +0200, Eli Cohen wrote:
> > > We should not try to use the VF MAC address as that is used by the
> > > regular (e.g. mlx5_core) NIC implementation. Instead, use a random
> > > generated MAC address.
> > > 
> > > Suggested by: Cindy Lu <lulu@...hat.com>
> > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> > > Signed-off-by: Eli Cohen <elic@...dia.com>
> > 
> > I didn't realise it's possible to use VF in two ways
> > with and without vdpa.
> 
> Using a VF you can create quite a few resources, e.g. send queues
> recieve queues, virtio_net queues etc. So you can possibly create
> several instances of vdpa net devices and nic net devices.
> 
> > Could you include a bit more description on the failure
> > mode?
> 
> Well, using the MAC address of the nic vport is wrong since that is the
> MAC of the regular NIC implementation of mlx5_core.

Right but ATM it doesn't coexist with vdpa so what's the problem?

> > Is switching to a random mac for such an unusual
> > configuration really justified?
> 
> Since I can't use the NIC's MAC address, I have two options:
> 1. To get the MAC address as was chosen by the user administering the
>    NIC. This should invoke the set_config callback. Unfortunately this
>    is not implemented yet.
> 
> 2. Use a random MAC address. This is OK since if (1) is implemented it
>    can always override this random configuration.
> 
> > It looks like changing a MAC could break some guests,
> > can it not?
> >
> 
> No, it will not. The current version of mlx5 VDPA does not allow regular
> NIC driver and VDPA to co-exist. I have patches ready that enable that
> from steering point of view. I will post them here once other patches on
> which they depend will be merged.
> 
> https://patchwork.ozlabs.org/project/netdev/patch/20201120230339.651609-12-saeedm@nvidia.com/

Could you be more explicit on the following points:
- which configuration is broken ATM (as in, two device have identical
  macs? any other issues)?
- why won't device MAC change from guest point of view?


> > > ---
> > >  drivers/vdpa/mlx5/net/mlx5_vnet.c | 5 +----
> > >  1 file changed, 1 insertion(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > index 1fa6fcac8299..80d06d958b8b 100644
> > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > @@ -1955,10 +1955,7 @@ void *mlx5_vdpa_add_dev(struct mlx5_core_dev *mdev)
> > >  	if (err)
> > >  		goto err_mtu;
> > >  
> > > -	err = mlx5_query_nic_vport_mac_address(mdev, 0, 0, config->mac);
> > > -	if (err)
> > > -		goto err_mtu;
> > > -
> > > +	eth_random_addr(config->mac);
> > >  	mvdev->vdev.dma_dev = mdev->device;
> > >  	err = mlx5_vdpa_alloc_resources(&ndev->mvdev);
> > >  	if (err)
> > > -- 
> > > 2.26.2
> >