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Subject: Re: [PATCH bpf-next v4] virtio_net: add XDP meta data support
To:     Jason Wang <jasowang@redhat.com>
Cc:     ast@kernel.org, daniel@iogearbox.net, davem@davemloft.net,
        hawk@kernel.org, john.fastabend@gmail.com, kafai@fb.com,
        mst@redhat.com, songliubraving@fb.com, yhs@fb.com, kuba@kernel.org,
        andriin@fb.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, bpf@vger.kernel.org
References: <8da1b560-3128-b885-b453-13de5c7431fb@redhat.com>
 <20200204071655.94474-1-yuya.kusakabe@gmail.com>
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From:   Yuya Kusakabe <yuya.kusakabe@gmail.com>
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Message-ID: <1100837f-075f-dc97-cd14-758c96f2ac1d@gmail.com>
Date:   Wed, 5 Feb 2020 18:18:13 +0900
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On 2/5/20 1:10 PM, Jason Wang wrote:
> 
> On 2020/2/4 下午3:16, Yuya Kusakabe wrote:
>> Implement support for transferring XDP meta data into skb for
>> virtio_net driver; before calling into the program, xdp.data_meta points
>> to xdp.data and copy vnet header to the front of xdp.data_hard_start
>> to avoid overwriting it, where on program return with pass verdict,
>> we call into skb_metadata_set().
>>
>> Fixes: de8f3a83b0a0 ("bpf: add meta pointer for direct access")
>> Signed-off-by: Yuya Kusakabe <yuya.kusakabe@gmail.com>
>> ---
>>   drivers/net/virtio_net.c | 47 ++++++++++++++++++++++++++++------------
>>   1 file changed, 33 insertions(+), 14 deletions(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 2fe7a3188282..5fdd6ea0e3f1 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -371,7 +371,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>>                      struct receive_queue *rq,
>>                      struct page *page, unsigned int offset,
>>                      unsigned int len, unsigned int truesize,
>> -                   bool hdr_valid)
>> +                   bool hdr_valid, unsigned int metasize)
>>   {
>>       struct sk_buff *skb;
>>       struct virtio_net_hdr_mrg_rxbuf *hdr;
>> @@ -393,7 +393,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>>       else
>>           hdr_padded_len = sizeof(struct padded_vnet_hdr);
>>   -    if (hdr_valid)
>> +    if (hdr_valid && !metasize)
> 
> 
> hdr_valid means no XDP, so I think we can remove the check for metasize here and add a comment instead?

I will fix it on next patch.

> 
>>           memcpy(hdr, p, hdr_len);
>>         len -= hdr_len;
>> @@ -405,6 +405,11 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>>           copy = skb_tailroom(skb);
>>       skb_put_data(skb, p, copy);
>>   +    if (metasize) {
>> +        __skb_pull(skb, metasize);
>> +        skb_metadata_set(skb, metasize);
>> +    }
>> +
>>       len -= copy;
>>       offset += copy;
>>   @@ -644,6 +649,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
>>       unsigned int delta = 0;
>>       struct page *xdp_page;
>>       int err;
>> +    unsigned int metasize = 0;
>>         len -= vi->hdr_len;
>>       stats->bytes += len;
>> @@ -683,10 +689,15 @@ static struct sk_buff *receive_small(struct net_device *dev,
>>             xdp.data_hard_start = buf + VIRTNET_RX_PAD + vi->hdr_len;
>>           xdp.data = xdp.data_hard_start + xdp_headroom;
>> -        xdp_set_data_meta_invalid(&xdp);
>>           xdp.data_end = xdp.data + len;
>> +        xdp.data_meta = xdp.data;
>>           xdp.rxq = &rq->xdp_rxq;
>>           orig_data = xdp.data;
>> +        /* Copy the vnet header to the front of data_hard_start to avoid
>> +         * overwriting it by XDP meta data.
>> +         */
>> +        memcpy(xdp.data_hard_start - vi->hdr_len,
>> +               xdp.data - vi->hdr_len, vi->hdr_len);
> 
> 
> I think we don't need this. And it looks to me there's a bug in the current code.
> 
> Commit 436c9453a1ac0 ("virtio-net: keep vnet header zeroed after processing XDP") leave the a corner case for receive_small() which still use:
> 
>         if (!delta) {
>                 buf += header_offset;
>                 memcpy(skb_vnet_hdr(skb), buf, vi->hdr_len);
>         } /* keep zeroed vnet hdr since packet was changed by bpf */
> 
> Which seems wrong, we need check xdp_prog instead of delta.
> 
> With this fixed, there's no need to care about the vnet header here since we don't know whether or not packet is modified by XDP.

I missed this commit. I understand this is the reason for "Awaiting Upstream".

> 
>>           act = bpf_prog_run_xdp(xdp_prog, &xdp);
>>           stats->xdp_packets++;
>>   @@ -695,9 +706,11 @@ static struct sk_buff *receive_small(struct net_device *dev,
>>               /* Recalculate length in case bpf program changed it */
>>               delta = orig_data - xdp.data;
>>               len = xdp.data_end - xdp.data;
>> +            metasize = xdp.data - xdp.data_meta;
>>               break;
>>           case XDP_TX:
>>               stats->xdp_tx++;
>> +            xdp.data_meta = xdp.data;
> 
> 
> I think we should remove the xdp_set_data_meta_invalid() at least? And move this initialization just after xdp.data is initialized.
> 
> Testing receive_small() requires to disable mrg_rxbuf, guest_tso4, guest_tso6 and guest_ufo from qemu command line.
> 
> 
>>               xdpf = convert_to_xdp_frame(&xdp);
>>               if (unlikely(!xdpf))
>>                   goto err_xdp;
>> @@ -736,10 +749,12 @@ static struct sk_buff *receive_small(struct net_device *dev,
>>       skb_reserve(skb, headroom - delta);
>>       skb_put(skb, len);
>>       if (!delta) {
> 
> 
> Need to check xdp_prog (need another patch).

I will fix it on next patch.

> 
> 
>> -        buf += header_offset;
>> -        memcpy(skb_vnet_hdr(skb), buf, vi->hdr_len);
>> +        memcpy(skb_vnet_hdr(skb), buf + VIRTNET_RX_PAD, vi->hdr_len);
>>       } /* keep zeroed vnet hdr since packet was changed by bpf */
>>   +    if (metasize)
>> +        skb_metadata_set(skb, metasize);
>> +
>>   err:
>>       return skb;
>>   @@ -760,8 +775,8 @@ static struct sk_buff *receive_big(struct net_device *dev,
>>                      struct virtnet_rq_stats *stats)
>>   {
>>       struct page *page = buf;
>> -    struct sk_buff *skb = page_to_skb(vi, rq, page, 0, len,
>> -                      PAGE_SIZE, true);
>> +    struct sk_buff *skb =
>> +        page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, true, 0);
>>         stats->bytes += len - vi->hdr_len;
>>       if (unlikely(!skb))
>> @@ -793,6 +808,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>>       unsigned int truesize;
>>       unsigned int headroom = mergeable_ctx_to_headroom(ctx);
>>       int err;
>> +    unsigned int metasize = 0;
>>         head_skb = NULL;
>>       stats->bytes += len - vi->hdr_len;
>> @@ -839,8 +855,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>>           data = page_address(xdp_page) + offset;
>>           xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len;
>>           xdp.data = data + vi->hdr_len;
>> -        xdp_set_data_meta_invalid(&xdp);
>>           xdp.data_end = xdp.data + (len - vi->hdr_len);
>> +        xdp.data_meta = xdp.data;
>>           xdp.rxq = &rq->xdp_rxq;
>>             act = bpf_prog_run_xdp(xdp_prog, &xdp);
>> @@ -852,8 +868,9 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>>                * adjustments. Note other cases do not build an
>>                * skb and avoid using offset
>>                */
>> -            offset = xdp.data -
>> -                    page_address(xdp_page) - vi->hdr_len;
>> +            metasize = xdp.data - xdp.data_meta;
>> +            offset = xdp.data - page_address(xdp_page) -
>> +                 vi->hdr_len - metasize;
>>                 /* recalculate len if xdp.data or xdp.data_end were
>>                * adjusted
>> @@ -863,14 +880,15 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>>               if (unlikely(xdp_page != page)) {
>>                   rcu_read_unlock();
>>                   put_page(page);
>> -                head_skb = page_to_skb(vi, rq, xdp_page,
>> -                               offset, len,
>> -                               PAGE_SIZE, false);
>> +                head_skb = page_to_skb(vi, rq, xdp_page, offset,
>> +                               len, PAGE_SIZE, false,
>> +                               metasize);
>>                   return head_skb;
>>               }
>>               break;
>>           case XDP_TX:
>>               stats->xdp_tx++;
>> +            xdp.data_meta = xdp.data;
> 
> 
> Any reason for doing this?

XDP_TX can not support metadata for now, because if metasize > 0, __virtnet_xdp_xmit_one() returns EOPNOTSUPP.

static int __virtnet_xdp_xmit_one(struct virtnet_info *vi,
				   struct send_queue *sq,
				   struct xdp_frame *xdpf)
{
	struct virtio_net_hdr_mrg_rxbuf *hdr;
	int err;

	/* virtqueue want to use data area in-front of packet */
	if (unlikely(xdpf->metasize > 0))
		return -EOPNOTSUPP;


> 
> Thanks
> 
> 
>>               xdpf = convert_to_xdp_frame(&xdp);
>>               if (unlikely(!xdpf))
>>                   goto err_xdp;
>> @@ -921,7 +939,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>>           goto err_skb;
>>       }
>>   -    head_skb = page_to_skb(vi, rq, page, offset, len, truesize, !xdp_prog);
>> +    head_skb = page_to_skb(vi, rq, page, offset, len, truesize, !xdp_prog,
>> +                   metasize);
>>       curr_skb = head_skb;
>>         if (unlikely(!curr_skb))
> 

Thank you for your kind review.