netdev - Re: [PATCH v2 6/9] virtio_net: transmit the multi-buffer xdp

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Date:   Wed, 28 Dec 2022 14:30:52 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Heng Qi <hengqi@...ux.alibaba.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     "Michael S . Tsirkin" <mst@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH v2 6/9] virtio_net: transmit the multi-buffer xdp


在 2022/12/27 16:26, Heng Qi 写道:
>
>
> 在 2022/12/27 下午3:12, Jason Wang 写道:
>>
>> 在 2022/12/20 22:14, Heng Qi 写道:
>>> This serves as the basis for XDP_TX and XDP_REDIRECT
>>> to send a multi-buffer xdp_frame.
>>>
>>> Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com>
>>> Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
>>> ---
>>>   drivers/net/virtio_net.c | 27 ++++++++++++++++++++++-----
>>>   1 file changed, 22 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index 40bc58fa57f5..9f31bfa7f9a6 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -563,22 +563,39 @@ static int __virtnet_xdp_xmit_one(struct 
>>> virtnet_info *vi,
>>>                      struct xdp_frame *xdpf)
>>>   {
>>>       struct virtio_net_hdr_mrg_rxbuf *hdr;
>>> -    int err;
>>> +    struct skb_shared_info *shinfo;
>>> +    u8 nr_frags = 0;
>>> +    int err, i;
>>>         if (unlikely(xdpf->headroom < vi->hdr_len))
>>>           return -EOVERFLOW;
>>>   -    /* Make room for virtqueue hdr (also change xdpf->headroom?) */
>>> +    if (unlikely(xdp_frame_has_frags(xdpf))) {
>>> +        shinfo = xdp_get_shared_info_from_frame(xdpf);
>>> +        nr_frags = shinfo->nr_frags;
>>> +    }
>>> +
>>> +    /* Need to adjust this to calculate the correct postion
>>> +     * for shinfo of the xdpf.
>>> +     */
>>> +    xdpf->headroom -= vi->hdr_len;
>>
>>
>> Any reason we need to do this here? (Or if it is, is it only needed 
>> for multibuffer XDP?)
>
> Going back to its wrapping function virtnet_xdp_xmit(), we need to 
> free up the pending old buffers.
> If the "is_xdp_frame(ptr)" condition is met, then we need to calculate 
> the position of skb_shared_info
> in xdp_get_frame_len() and xdp_return_frame(), which will involve to 
> xdpf->data and xdpf->headroom.
> Therefore, we need to update the value of headroom synchronously here.


Let's tweak the comment above to something like this.

With that fixed,

Acked-by: Jason Wang <jasowang@...hat.com>

Thanks


>
> Also, it's not necessary for single-buffer xdp, but we need to keep it 
> because it's harmless and as it should be.
>
> Thanks.
>
>>
>> Other looks good.
>>
>> Thanks
>>
>>
>>>       xdpf->data -= vi->hdr_len;
>>>       /* Zero header and leave csum up to XDP layers */
>>>       hdr = xdpf->data;
>>>       memset(hdr, 0, vi->hdr_len);
>>>       xdpf->len   += vi->hdr_len;
>>>   -    sg_init_one(sq->sg, xdpf->data, xdpf->len);
>>> +    sg_init_table(sq->sg, nr_frags + 1);
>>> +    sg_set_buf(sq->sg, xdpf->data, xdpf->len);
>>> +    for (i = 0; i < nr_frags; i++) {
>>> +        skb_frag_t *frag = &shinfo->frags[i];
>>> +
>>> +        sg_set_page(&sq->sg[i + 1], skb_frag_page(frag),
>>> +                skb_frag_size(frag), skb_frag_off(frag));
>>> +    }
>>>   -    err = virtqueue_add_outbuf(sq->vq, sq->sg, 1, xdp_to_ptr(xdpf),
>>> -                   GFP_ATOMIC);
>>> +    err = virtqueue_add_outbuf(sq->vq, sq->sg, nr_frags + 1,
>>> +                   xdp_to_ptr(xdpf), GFP_ATOMIC);
>>>       if (unlikely(err))
>>>           return -ENOSPC; /* Caller handle free/refcnt */
>