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

[Eli Cohen]

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@xxxxxxxxxx>
> > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> > Signed-off-by: Eli Cohen <elic@xxxxxxxxxx>
> 
> 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.

> 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@xxxxxxxxxx/
 
> > ---
> >  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
>