Date:   Wed, 28 Dec 2022 16:25:17 +0800
From:   Heng Qi <hengqi@...ux.alibaba.com>
To:     Jason Wang <jasowang@...hat.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 2022/12/28 下午2:30, Jason Wang 写道:
>
> 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.

Ok, thanks for your energy.

>
> 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 */
>>

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