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

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Message-ID: <55b7ae23-6000-4699-9bac-5e72fbdcd803@nvidia.com>
Date: Thu, 29 Aug 2024 12:00:22 +0200
From: Dragos Tatulea <dtatulea@...dia.com>
To: Cindy Lu <lulu@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>, si-wei.liu@...cle.com, Jiri Pirko <jiri@...dia.com>,
 virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vdpa/mlx5: Use random MAC address when no nic vport MAC
 set



On 29.08.24 11:05, Cindy Lu wrote:
> On Wed, 28 Aug 2024 at 17:37, Dragos Tatulea <dtatulea@...dia.com> wrote:
>>
>>
>>
>> On 28.08.24 11:00, Cindy Lu wrote:
>>> On Wed, 28 Aug 2024 at 09:51, Jason Wang <jasowang@...hat.com> wrote:
>>>>
>>>> On Wed, Aug 28, 2024 at 12:03 AM Dragos Tatulea <dtatulea@...dia.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@...dia.com>
>>>>> Reviewed-by: Jiri Pirko <jiri@...dia.com>
>>>>
>>>> Acked-by: Jason Wang <jasowang@...hat.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.

Thanks,
Dragos