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

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Date:   Tue, 27 Dec 2022 16:26:09 +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 2022/12/27 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.

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 */