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SubjectRe: [PATCH] vdpa/mlx5: Use random MAC for the vdpa net instance

On 2020/12/1 下午5:23, Cindy Lu wrote:
> On Mon, Nov 30, 2020 at 11:33 PM Michael S. Tsirkin <> wrote:
>> On Mon, Nov 30, 2020 at 06:41:45PM +0800, Cindy Lu wrote:
>>> On Mon, Nov 30, 2020 at 5:33 PM Michael S. Tsirkin <> wrote:
>>>> On Mon, Nov 30, 2020 at 11:27:59AM +0200, Eli Cohen wrote:
>>>>> On Mon, Nov 30, 2020 at 04:00:51AM -0500, Michael S. Tsirkin wrote:
>>>>>> On Mon, Nov 30, 2020 at 08:27:46AM +0200, Eli Cohen wrote:
>>>>>>> On Sun, Nov 29, 2020 at 03:08:22PM -0500, Michael S. Tsirkin wrote:
>>>>>>>> On Sun, Nov 29, 2020 at 08:43:51AM +0200, Eli Cohen wrote:
>>>>>>>>> We should not try to use the VF MAC address as that is used by the
>>>>>>>>> regular (e.g. mlx5_core) NIC implementation. Instead, use a random
>>>>>>>>> generated MAC address.
>>>>>>>>> Suggested by: Cindy Lu <>
>>>>>>>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
>>>>>>>>> Signed-off-by: Eli Cohen <>
>>>>>>>> I didn't realise it's possible to use VF in two ways
>>>>>>>> with and without vdpa.
>>>>>>> Using a VF you can create quite a few resources, e.g. send queues
>>>>>>> recieve queues, virtio_net queues etc. So you can possibly create
>>>>>>> several instances of vdpa net devices and nic net devices.
>>>>>>>> Could you include a bit more description on the failure
>>>>>>>> mode?
>>>>>>> Well, using the MAC address of the nic vport is wrong since that is the
>>>>>>> MAC of the regular NIC implementation of mlx5_core.
>>>>>> Right but ATM it doesn't coexist with vdpa so what's the problem?
>>>>> This call is wrong: mlx5_query_nic_vport_mac_address()
>>>>>>>> Is switching to a random mac for such an unusual
>>>>>>>> configuration really justified?
>>>>>>> Since I can't use the NIC's MAC address, I have two options:
>>>>>>> 1. To get the MAC address as was chosen by the user administering the
>>>>>>> NIC. This should invoke the set_config callback. Unfortunately this
>>>>>>> is not implemented yet.
>>>>>>> 2. Use a random MAC address. This is OK since if (1) is implemented it
>>>>>>> can always override this random configuration.
>>>>>>>> It looks like changing a MAC could break some guests,
>>>>>>>> can it not?
>>>>>>> No, it will not. The current version of mlx5 VDPA does not allow regular
>>>>>>> NIC driver and VDPA to co-exist. I have patches ready that enable that
>>>>>>> from steering point of view. I will post them here once other patches on
>>>>>>> which they depend will be merged.
>>>>>> Could you be more explicit on the following points:
>>>>>> - which configuration is broken ATM (as in, two device have identical
>>>>>> macs? any other issues)?
>>>>> The only wrong thing is the call to mlx5_query_nic_vport_mac_address().
>>>>> It's not breaking anything yet is wrong. The random MAC address setting
>>>>> is required for the steering patches.
>>>> Okay so I'm not sure the Fixes tag at least is appropriate if it's a
>>>> dependency of a new feature.
>>>>>> - why won't device MAC change from guest point of view?
>>>>> It's lack of implementation in qemu as far as I know.
>>>> Sorry not sure I understand. What's not implemented in QEMU?
>>> HI Michael, there are some bug in qemu to set_config, this will fix in future,
>>> But this patch is still needed, because without this patch the mlx
>>> driver will give an 0 mac address to qemu
>>> and qemu will overwrite the default mac address. This will cause traffic down.
>> Hmm the patch description says VF mac address, not 0 address. Confused.
>> If there's no mac we can clear VIRTIO_NET_F_MAC and have guest
>> use a random value ...

I'm not sure this can work for all types of vDPA (e.g it could not be a
learning bridge in the swtich).

> hi Michael,
> I have tried as your suggestion, seems even remove the
> VIRTIO_NET_F_MAC the qemu will still call get_cinfig and overwrite the
> default address in VM,

This looks a bug in qemu, in guest driver we had:

    /* Configuration may specify what MAC to use.  Otherwise random. */
    if (virtio_has_feature(vdev, VIRTIO_NET_F_MAC))
                   offsetof(struct virtio_net_config, mac),
                   dev->dev_addr, dev->addr_len);

> this process is like
> vdpa _init -->qemu call get_config ->mlx driver will give an mac
> address with all 0-->
> qemu will not check this mac address and use it --> overwrite the mac
> address in qemu
> So for my understanding there are several method to fix this problem
> 1, qemu check the mac address, if the mac address is all 0, qemu will
> ignore it and set the random mac address to mlx driver.

So my understanding is that, if mac address is all 0, vDPA parent should
not advertise VIRTIO_NET_F_MAC. And qemu should emulate this feature as
you did:

1) get a random mac
2) advertise VIRTIO_NET_F_MAC
3) set the random mac to vDPA through set_config
4) advertise the random mac to emulated config to guest

> 2. mlx driver checks the mac address and if this mac is 0, return fail
> to qemu, but this need to change the UAPI.

uAPI is probably fine since ioctl can fail.  We can change the to allow
the set_config to fail but virito spec doesn't have a way to advertise
the error in this case. Anyway, the driver only risk itself for setting
a wrong value, so we're probably fine.


> 3. mlx driver it shelf should get an correct mac address while it init.
> 4. add check in qemu get_config function , if there is not F_MAC Then
> ignore the mac address from mlx driver
> not sure which method is more suitable ?
> Thanks
> Cindy
>>>>>>>>> ---
>>>>>>>>> drivers/vdpa/mlx5/net/mlx5_vnet.c | 5 +----
>>>>>>>>> 1 file changed, 1 insertion(+), 4 deletions(-)
>>>>>>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>> index 1fa6fcac8299..80d06d958b8b 100644
>>>>>>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>> @@ -1955,10 +1955,7 @@ void *mlx5_vdpa_add_dev(struct mlx5_core_dev *mdev)
>>>>>>>>> if (err)
>>>>>>>>> goto err_mtu;
>>>>>>>>> - err = mlx5_query_nic_vport_mac_address(mdev, 0, 0, config->mac);
>>>>>>>>> - if (err)
>>>>>>>>> - goto err_mtu;
>>>>>>>>> -
>>>>>>>>> + eth_random_addr(config->mac);
>>>>>>>>> mvdev->vdev.dma_dev = mdev->device;
>>>>>>>>> err = mlx5_vdpa_alloc_resources(&ndev->mvdev);
>>>>>>>>> if (err)
>>>>>>>>> --
>>>>>>>>> 2.26.2

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