Message-ID: <82a9a043-25de-4ac5-965b-3902b3a179a9@nvidia.com>
Date: Mon, 2 Sep 2024 11:10:42 +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 02.09.24 10:53, Dragos Tatulea wrote:
> 
> 
> On 02.09.24 10:40, Cindy Lu wrote:
>> On Fri, 30 Aug 2024 at 22:46, Dragos Tatulea <dtatulea@...dia.com> wrote:
>>>
>>> Hi Cindy,
>>>
>>> On 30.08.24 15:52, Dragos Tatulea wrote:
>>>>
>>>>
>>>> On 30.08.24 11:12, Cindy Lu wrote:
>>>>> On Thu, 29 Aug 2024 at 18:00, Dragos Tatulea <dtatulea@...dia.com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> 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
>>>>>>
>>>>> Hi Dragos，
>>>>>  I think we need to hold this patch. Because it may not be working
>>>>> with upstream qemu.
>>>>>
>>>>> MLX will create a random MAC address for your patch. Additionally, if
>>>>> there is no specific MAC in the QEMU command line, QEMU will also
>>>>> generate a random MAC.
>>>>> these two MAC are not the same. and this will cause traffic loss.
>>>> Ahaa, it turns out that qemu 8.x and 9.x have different behaviour.
>>>>
>>>> Initially I was testing this scenario (vdpa device created with no mac
>>>> and no mac set in qemu cli) with qemu 8.x. There, qemu was not being
>>>> able to set the qemu generated random mac addres because .set_config()
>>>> is a nop in mlx5_vdpa.
>>>>
>>>> Then I moved to qemu 9.x and saw that this scenario was working because
>>>> now the CVQ was used instead to configure the mac on the device.
>>>>
>>>> So this patch should definitely not be applied.
>>>>
>>>> I was thinking if there are ways to fix this for 8.x. The only feasible
>>>> way is to implement .set_config() in mlx5_vdpa for the mac
>>>> configuration. But as you previousy said, this is discouraged.
>>>>
>>> I just tested your referenced qemu fix from patchwork and I found that
>>> for the case when a vdpa device doesn't have a mac address (mac address
>>> 0 and VIRTIO_NET_F_MAC not set) qemu will return an error. So with this
>>> fix we'd be back to square one where the user always has to set a mac
>>> somewhere.
>>>
>>> Would it be possible to take this case into consideration with your
>>> fix?
>>>
>>> Thanks,
>>> Dragos
>>>
>> Hi Dragos
>>
>> Thanks for your test and help, I think I can add a check for
>> VIRTIO_NET_F_MAC in the qemu code. if the device's Mac is 0 and the
>> VIRTIO_NET_F_MAC is not set. The guest VM will fail to load. I will
>> double-check this
> My request was to use the random MAC from qemu in this case. qemu is
> able to configure the device via CVQ. At least this device...
Also, it makes sense that if a device doesn't have a MAC set and
VIRTIO_NET_F_CTRL_MAC_ADDR is set then qemu can set a random mac on
that device. 

Thanks,
Dragos

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