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

[Dragos Tatulea <dtatulea@nvidia.com>]

On 29.08.24 12:00, Dragos Tatulea wrote:
> 
> 
> On 29.08.24 11:05, Cindy Lu wrote:
>> On Wed, 28 Aug 2024 at 17:37, Dragos Tatulea <dtatulea@nvidia.com> wrote:
>>>
>>>
>>>
>>> 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.
>>>
>> this is not a new bug. We are trying to fix it because it will cause
>> traffic lose without any warning.
>> in my fix , this setting (different mac in device and Qemu) will fail
>> to load the VM.
> Which is a good thing, right? Some feedback to the user that there is
> a misconfig. I got bitten by this so many times... Thank you for adding it.
> 
>>
>>>> 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.
>>>
>> I do not mean your patch will not work, I just want to make some
>> clarify here.Your patch + my fix may cause the VM to fail to load in
>> some situations, and this is as expected.
>> Your patch is good to merge.
> Ack. Thank you for the clarification.
(Side note)
While looking at another issue I discovered that it's possible to
configure a random MAC on the mlx5_vdpa device at VM boot time if
device MAC configuration is implemented during during .set_config(). So
I was able to boot up a VM with a random MAC address coming from qemu
and the traffic worked with this new MAC.

So now I'm not sure if this is just by luck or if the .set_config()
op should be implemented for the MAC part in our device.

Thanks,
Dragos