netdev - Re: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace when VIRTIO_NET_F

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Date:   Wed, 13 Jul 2022 15:47:40 +0800
From:   "Zhu, Lingshan" <lingshan.zhu@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Parav Pandit <parav@...dia.com>
Cc:     "jasowang@...hat.com" <jasowang@...hat.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xieyongji@...edance.com" <xieyongji@...edance.com>,
        "gautam.dawar@....com" <gautam.dawar@....com>
Subject: Re: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace
 when VIRTIO_NET_F_MQ == 0



On 7/13/2022 1:26 PM, Michael S. Tsirkin wrote:
> On Fri, Jul 01, 2022 at 10:07:59PM +0000, Parav Pandit wrote:
>>
>>> From: Zhu Lingshan <lingshan.zhu@...el.com>
>>> Sent: Friday, July 1, 2022 9:28 AM
>>> If VIRTIO_NET_F_MQ == 0, the virtio device should have one queue pair, so
>>> when userspace querying queue pair numbers, it should return mq=1 than
>>> zero.
>>>
>>> Function vdpa_dev_net_config_fill() fills the attributions of the vDPA
>>> devices, so that it should call vdpa_dev_net_mq_config_fill() so the
>>> parameter in vdpa_dev_net_mq_config_fill() should be feature_device than
>>> feature_driver for the vDPA devices themselves
>>>
>>> Before this change, when MQ = 0, iproute2 output:
>>> $vdpa dev config show vdpa0
>>> vdpa0: mac 00:e8:ca:11:be:05 link up link_announce false max_vq_pairs 0
>>> mtu 1500
>>>
>> The fix belongs to user space.
>> When a feature bit _MQ is not negotiated, vdpa kernel space will not add attribute VDPA_ATTR_DEV_NET_CFG_MAX_VQP.
>> When such attribute is not returned by kernel, max_vq_pairs should not be shown by the iproute2.
>>
>> We have many config space fields that depend on the feature bits and some of them do not have any defaults.
>> To keep consistency of existence of config space fields among all, we don't want to show default like below.
>>
>> Please fix the iproute2 to not print max_vq_pairs when it is not returned by the kernel.
> Parav I read the discussion and don't get your argument. From driver's POV
> _MQ with 1 VQ pair and !_MQ are exactly functionally equivalent.
>
> It's true that iproute probably needs to be fixed too, to handle old
> kernels. But iproute is not the only userspace, why not make it's life
> easier by fixing the kernel?
I will fix iproute2 once this series settles down

Thanks,
Zhu Lingshan