Re: [PATCH] vdpa/mlx5: Use random MAC address when no nic vport MAC set

[Dragos Tatulea <dtatulea@nvidia.com>]

On 28.08.24 11:00, Cindy Lu wrote:
> On Wed, 28 Aug 2024 at 09:51, Jason Wang <jasowang@redhat.com> wrote:
>>
>> On Wed, Aug 28, 2024 at 12:03 AM Dragos Tatulea <dtatulea@nvidia.com> wrote:
>>>
>>> When the vdpa device is configured without a specific MAC
>>> address, the vport MAC address is used. However, this
>>> address can be 0 which prevents the driver from properly
>>> configuring the MPFS and breaks steering.
>>>
>>> The solution is to simply generate a random MAC address
>>> when no MAC is set on the nic vport.
>>>
>>> Now it's possible to create a vdpa device without a
>>> MAC address and run qemu with this device without needing
>>> to configure an explicit MAC address.
>>>
>>> Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
>>> Reviewed-by: Jiri Pirko <jiri@nvidia.com>
>>
>> Acked-by: Jason Wang <jasowang@redhat.com>
>>
>> (Adding Cindy for double checking if it has any side effect on Qemu side)
>>
>> Thanks
>>
> But Now there is a bug in QEMU: if the hardware MAC address does not
> match the one in the QEMU command line, it will cause traffic loss.
> 
Why is this a new issue in qemu? qemu in it's current state won't work
with a different mac address that the one that is set in HW anyway.

> So, Just an FYI here: if your patch merged, it may cause traffic loss.
> and now I'm working in the fix it in qemu, the link is
> https://patchew.org/QEMU/20240716011349.821777-1-lulu@redhat.com/
> The idea of this fix is
> There are will only two acceptable situations for qemu:
> 1. The hardware MAC address is the same as the MAC address specified
> in the QEMU command line, and both MAC addresses are not 0.
> 2. The hardware MAC address is not 0, and the MAC address in the QEMU
> command line is 0. In this situation, the hardware MAC address will
> overwrite the QEMU command line address.
> 
Why would this not work with this patch? This patch simply sets a MAC
if the vport doesn't have one set. Which allows for more scenarios to
work.

Thanks,
Dragos

> Thanks
> Cindy
> 
> 
>>> ---
>>>  drivers/vdpa/mlx5/net/mlx5_vnet.c | 3 +++
>>>  1 file changed, 3 insertions(+)
>>>
>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> index fa78e8288ebb..1c26139d02fe 100644
>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> @@ -3824,6 +3824,9 @@ static int mlx5_vdpa_dev_add(struct vdpa_mgmt_dev *v_mdev, const char *name,
>>>                 err = mlx5_query_nic_vport_mac_address(mdev, 0, 0, config->mac);
>>>                 if (err)
>>>                         goto err_alloc;
>>> +
>>> +               if (is_zero_ether_addr(config->mac))
>>> +                       eth_random_addr(config->mac);
>>>         }
>>>
>>>         if (!is_zero_ether_addr(config->mac)) {
>>> --
>>> 2.45.1
>>>
>>
>