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Subject: Re: [PATCH] vdpa/mlx5: Use random MAC for the vdpa net instance
To:     "Michael S. Tsirkin" <mst@redhat.com>, Eli Cohen <elic@nvidia.com>
Cc:     Cindy Lu <lulu@redhat.com>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <3e32ef6d-83c9-5866-30e5-f6eeacd5044d@redhat.com>
Date:   Wed, 2 Dec 2020 21:48:25 +0800
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On 2020/12/2 下午5:23, Michael S. Tsirkin wrote:
> On Wed, Dec 02, 2020 at 07:57:14AM +0200, Eli Cohen wrote:
>> On Wed, Dec 02, 2020 at 12:18:36PM +0800, Jason Wang wrote:
>>> On 2020/12/1 下午5:23, Cindy Lu wrote:
>>>> On Mon, Nov 30, 2020 at 11:33 PM Michael S. Tsirkin<mst@redhat.com>  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<mst@redhat.com>  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<lulu@redhat.com>
>>>>>>>>>>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
>>>>>>>>>>>> Signed-off-by: Eli Cohen<elic@nvidia.com>
>>>>>>>>>>> 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.
>>>>>>>>>>
>>>>>>>>>> https://patchwork.ozlabs.org/project/netdev/patch/20201120230339.651609-12-saeedm@nvidia.com/
>>>>>>>>> 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))
>>>          virtio_cread_bytes(vdev,
>>>                     offsetof(struct virtio_net_config, mac),
>>>                     dev->dev_addr, dev->addr_len);
>>>      else
>>>          eth_hw_addr_random(dev);
>>>
>>>
>>>> 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:
>> Thinking it over, at least in mlx5, I should always advertise
>> VIRTIO_NET_F_MAC and set a non zero MAC value. The source of the MAC can
>> be either randomly generated value by mlx5_vdpa or by a management tool.
>> This is important becauase we should not let the VM modify the MAC. If
>> we do it can set a MAC value identical to the mlx5 NIC driver and can
>> kidnap traffic that was not destined to it.
>>
>> In addition, when VIRTIO_NET_F_MAC is published, attempts to change the
>> MAC address from the VM should result in error.
> That is not what the spec says though.
> VIRTIO_NET_F_MAC only says whether mac is valid in the config space.
> Whether guest can control that depends on VIRTIO_NET_F_CTRL_MAC_ADDR:
>
> 	The VIRTIO_NET_CTRL_MAC_ADDR_SET command is used to set the default MAC address which rx
> 	filtering accepts (and if VIRTIO_NET_F_MAC_ADDR has been negotiated, this will be reflected in mac in
> 	config space).
> 	The command-specific-data for VIRTIO_NET_CTRL_MAC_ADDR_SET is the 6-byte MAC address.


Consider VIRTIO_NET_CTRL_MAC_ADDR_SET is not supported now. What Eli 
proposed here should work?

Thanks


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