netdev - Re: [PATCH v2 1/1] virtio_net: notify MAC address change on device initialization

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Date:   Wed, 25 Jan 2023 09:13:49 +0200
From:   Eli Cohen <elic@...dia.com>
To:     Laurent Vivier <lvivier@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Gautam Dawar <gautam.dawar@...inx.com>,
        Eugenio Pérez <eperezma@...hat.com>,
        netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        Cindy Lu <lulu@...hat.com>, Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Parav Pandit <parav@...dia.com>
Subject: Re: [PATCH v2 1/1] virtio_net: notify MAC address change on device
 initialization


On 24/01/2023 13:04, Laurent Vivier wrote:
> On 1/24/23 11:15, Michael S. Tsirkin wrote:
>> On Mon, Jan 23, 2023 at 01:00:22PM +0100, Laurent Vivier wrote:
>>> In virtnet_probe(), if the device doesn't provide a MAC address the
>>> driver assigns a random one.
>>> As we modify the MAC address we need to notify the device to allow it
>>> to update all the related information.
>>>
>>> The problem can be seen with vDPA and mlx5_vdpa driver as it doesn't
>>> assign a MAC address by default. The virtio_net device uses a random
>>> MAC address (we can see it with "ip link"), but we can't ping a net
>>> namespace from another one using the virtio-vdpa device because the
>>> new MAC address has not been provided to the hardware.
>>
>> And then what exactly happens? Does hardware drop the outgoing
>> or the incoming packets? Pls include in the commit log.
>
> I don't know. There is nothing in the kernel logs.
>
> The ping error is: "Destination Host Unreachable"
>
> I found the problem with the mlx5 driver as in "it doesn't work when 
> MAC address is not set"...
>
> Perhaps Eli can explain what happens when the MAC address is not set?

If the MAC address is changed without letting mlx5_vdpa know, RX packets 
will be dropped since they won't go through the receive filters.

TX packets will go through unaffected.

>
>>
>>> Signed-off-by: Laurent Vivier <lvivier@...hat.com>
>>> ---
>>>   drivers/net/virtio_net.c | 14 ++++++++++++++
>>>   1 file changed, 14 insertions(+)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index 7723b2a49d8e..4bdc8286678b 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -3800,6 +3800,8 @@ static int virtnet_probe(struct virtio_device 
>>> *vdev)
>>>           eth_hw_addr_set(dev, addr);
>>>       } else {
>>>           eth_hw_addr_random(dev);
>>> +        dev_info(&vdev->dev, "Assigned random MAC address %pM\n",
>>> +             dev->dev_addr);
>>>       }
>>>         /* Set up our device-specific information */
>>> @@ -3956,6 +3958,18 @@ static int virtnet_probe(struct virtio_device 
>>> *vdev)
>>>       pr_debug("virtnet: registered device %s with %d RX and TX 
>>> vq's\n",
>>>            dev->name, max_queue_pairs);
>>>   +    /* a random MAC address has been assigned, notify the device */
>>> +    if (!virtio_has_feature(vdev, VIRTIO_NET_F_MAC) &&
>>> +        virtio_has_feature(vi->vdev, VIRTIO_NET_F_CTRL_MAC_ADDR)) {
>>
>> Maybe add a comment explaining that we don't fail probe if
>> VIRTIO_NET_F_CTRL_MAC_ADDR is not there because
>> many devices work fine without getting MAC explicitly.
>
> OK
>
>>
>>> +        struct scatterlist sg;
>>> +
>>> +        sg_init_one(&sg, dev->dev_addr, dev->addr_len);
>>> +        if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_MAC,
>>> +                      VIRTIO_NET_CTRL_MAC_ADDR_SET, &sg)) {
>>> +            dev_warn(&vdev->dev, "Failed to update MAC address.\n");
>>
>> Here, I'm not sure we want to proceed. Is it useful sometimes?
>
> I think reporting an error is always useful, but I can remove that if 
> you prefer.
>
>> I note that we deny with virtnet_set_mac_address.
>>
>>> +        }
>>> +    }
>>> +
>>>       return 0;
>>
>>
>>
>> Also, some code duplication with virtnet_set_mac_address here.
>>
>> Also:
>>     When using the legacy interface, \field{mac} is driver-writable
>>     which provided a way for drivers to update the MAC without
>>     negotiating VIRTIO_NET_F_CTRL_MAC_ADDR.
>>
>> How about factoring out code in virtnet_set_mac_address
>> and reusing that?
>>
>
> In fact, we can write in the field only if we have VIRTIO_NET_F_MAC 
> (according to virtnet_set_mac_address(), and this code is executed 
> only if we do not have VIRTIO_NET_F_MAC. So I think it's better not 
> factoring the code as we have only the control queue case to manage.
>
>> This will also handle corner cases such as VIRTIO_NET_F_STANDBY
>> which are not currently addressed.
>
> F_STANDBY is only enabled when virtio-net device MAC address is equal 
> to the VFIO device MAC address, I don't think it can be enabled when 
> the MAC address is randomly assigned (in this case it has already 
> failed in net_failover_create(), as it has been called using the 
> random mac address), it's why I didn't check for it.
>
>>
>>
>>>   free_unregister_netdev:
>>> -- 
>>> 2.39.0
>>
>
> Thanks,
> Laurent
>