Re: VDPA MAC address problem

[Konstantin Shkolnyy <kshk@linux.ibm.com>]

On 3/20/2025 20:22, Jason Wang wrote:
> On Fri, Mar 21, 2025 at 12:45 AM Konstantin Shkolnyy <kshk@linux.ibm.com> wrote:
>>
>> On 3/19/2025 19:58, Jason Wang wrote:
>>> On Thu, Mar 20, 2025 at 12:34 AM Konstantin Shkolnyy <kshk@linux.ibm.com> wrote:
>>>> Upon reading this forum, I see that VHOST_VDPA_SET_CONFIG is
>>>> “deprecated”, and so VIRTIO_NET_CTRL_MAC_ADDR_SET must be the right
>>>> method, but it’s apparently called too late.
>>>
>>> VHOST_VDPA_SET_CONFIG requires the vDPA parent support which is not
>>> necessarily there.
>>
>> The mlx5 driver doesn't do anything for VHOST_VDPA_SET_CONFIG. Intel's
>> driver, however, apparently stores the configuration. So, it appears,
>> Intel will avoid the problem... Perhaps mlx5 could do the same so that
>> QEMU can set the address before it starts the VM (QEMU doesn't have to
>> later let the VM change the config).
> 
> The problem is that config space is not allowed to be written, that is
> why mlx5 doesn't implement the write method.
> 
>> Conceptually, setting the address
>> by QEMU cmdline doesn't look different from setting it by "vdpa dev add".
> 
> It's kind of different.
> 
> E.g the device may not even have VIRTIO_NET_F_MAC feature etc.

I'm not sure I understand... This is the MAC address returned by 
VHOST_VDPA_GET_CONFIG. If mlx5 allows it to be set by "vdpa dev add", 
shouldn't it also allow it to be set by VHOST_VDPA_SET_CONFIG called by 
virtio_net_device_realize(), before the VM exists.

When VM starts running, it queries this MAC address and QEMU fetches it 
by VHOST_VDPA_GET_CONFIG. But, because VHOST_VDPA_SET_CONFIG hasn't 
stored it, it fetches a wrong stale address.