netdev - Re: [RFC PATCH net-next v2 2/2] virtio_net: Extend virtio to use VF datapath when available

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Date:   Mon, 29 Jan 2018 11:32:53 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>, mst@...hat.com,
        stephen@...workplumber.org, davem@...emloft.net,
        netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        virtio-dev@...ts.oasis-open.org, jesse.brandeburg@...el.com,
        alexander.h.duyck@...el.com, kubakici@...pl
Subject: Re: [RFC PATCH net-next v2 2/2] virtio_net: Extend virtio to use VF
 datapath when available



On 2018年01月24日 00:03, Samudrala, Sridhar wrote:
>
>
> On 1/23/2018 2:33 AM, Jason Wang wrote:
>>
>>
>> On 2018年01月12日 13:58, Sridhar Samudrala wrote:
>>>   static netdev_tx_t start_xmit(struct sk_buff *skb, struct 
>>> net_device *dev)
>>>   {
>>>       struct virtnet_info *vi = netdev_priv(dev);
>>>       int qnum = skb_get_queue_mapping(skb);
>>>       struct send_queue *sq = &vi->sq[qnum];
>>> +    struct net_device *vf_netdev;
>>>       int err;
>>>       struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
>>>       bool kick = !skb->xmit_more;
>>>       bool use_napi = sq->napi.weight;
>>>   +    /* If VF is present and up then redirect packets
>>> +     * called with rcu_read_lock_bh
>>> +     */
>>> +    vf_netdev = rcu_dereference_bh(vi->vf_netdev);
>>> +    if (vf_netdev && netif_running(vf_netdev) &&
>>> +        !netpoll_tx_running(dev) &&
>>> +        is_unicast_ether_addr(eth_hdr(skb)->h_dest))
>>> +        return virtnet_vf_xmit(dev, vf_netdev, skb);
>>> +
>>
>> A question here.
>>
>> If I read the code correctly, all features were validated against 
>> virtio instead VF before transmitting. This assumes VF's feature is a 
>> superset of virtio, does this really work? Do we need to sanitize the 
>> feature before joining? (e.g at last NETIF_R_GSO_ROBUST needs to be 
>> removed).
>
> Actually, virtnet_vf_xmit() calls dev_queue_xmit() after updating 
> skb->dev to vf netdev.
> So the features get validated against VF features and the right tx 
> queue is selected
> before the real transmit.

I see, the the packet will go through qdiscs twice which is suboptimal. 
And the device may lose some ability of VF like tunnel GSO.

Thanks

>
> Thanks
> Sridhar
>
>
>