From: Jason Wang <jasowang@redhat.com>
To: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: mst@redhat.com, virtualization@lists.linux-foundation.org,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
bpf@vger.kernel.org, "Jubran, Samih" <sameehj@amazon.com>
Subject: Re: [PATCH net-next 1/2] virtio-net: don't reserve space for vnet header for XDP
Date: Wed, 6 May 2020 16:34:36 +0800 [thread overview]
Message-ID: <3ecb558b-5281-2497-db3c-6aae7d7f882b@redhat.com> (raw)
In-Reply-To: <20200506102123.739f1233@carbon>
On 2020/5/6 下午4:21, Jesper Dangaard Brouer wrote:
> On Wed, 6 May 2020 14:16:32 +0800
> Jason Wang <jasowang@redhat.com> wrote:
>
>> We tried to reserve space for vnet header before
>> xdp.data_hard_start. But this is useless since the packet could be
>> modified by XDP which may invalidate the information stored in the
>> header and
> IMHO above statements are wrong. XDP cannot access memory before
> xdp.data_hard_start. Thus, it is safe to store a vnet headers before
> xdp.data_hard_start. (The sfc driver also use this "before" area).
The problem is if we place vnet header before data_hard_start,
virtio-net will fail any header adjustment.
Or do you mean to copy vnet header before data_hard_start before
processing XDP?
>
>> there's no way for XDP to know the existence of the vnet header currently.
> It is true that XDP is unaware of this area, which is the way it
> should be. Currently the area will survive after calling BPF/XDP.
> After your change it will be overwritten in xdp_frame cases.
>
>
>> So let's just not reserve space for vnet header in this case.
> I think this is a wrong approach!
>
> We are working on supporting GRO multi-buffer for XDP. The vnet header
> contains GRO information (see pahole below sign).
Another note is that since we need reserve room for skb_shared_info, GRO
for XDP may probably lead more frag list.
> It is currently not
> used in the XDP case, but we will be working towards using it.
Good to know that, but I think it can only work when the packet is not
modified by XDP?
> There
> are a lot of unanswered questions on how this will be implemented.
> Thus, I cannot layout how we are going to leverage this info yet, but
> your patch are killing this info, which IHMO is going in the wrong
> direction.
I can copy vnet header ahead of data_hard_start, does it work for you?
Thanks
>
>
>> Cc: Jesper Dangaard Brouer <brouer@redhat.com>
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>> ---
>> drivers/net/virtio_net.c | 6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 11f722460513..98dd75b665a5 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -684,8 +684,8 @@ static struct sk_buff *receive_small(struct net_device *dev,
>> page = xdp_page;
>> }
>>
>> - xdp.data_hard_start = buf + VIRTNET_RX_PAD + vi->hdr_len;
>> - xdp.data = xdp.data_hard_start + xdp_headroom;
>> + xdp.data_hard_start = buf + VIRTNET_RX_PAD;
>> + xdp.data = xdp.data_hard_start + xdp_headroom + vi->hdr_len;
>> xdp.data_end = xdp.data + len;
>> xdp.data_meta = xdp.data;
>> xdp.rxq = &rq->xdp_rxq;
>> @@ -845,7 +845,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>> * the descriptor on if we get an XDP_TX return code.
>> */
>> data = page_address(xdp_page) + offset;
>> - xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len;
>> + xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM;
>> xdp.data = data + vi->hdr_len;
>> xdp.data_end = xdp.data + (len - vi->hdr_len);
>> xdp.data_meta = xdp.data;
>
>
next prev parent reply other threads:[~2020-05-06 8:34 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-06 6:16 [PATCH net-next 1/2] virtio-net: don't reserve space for vnet header for XDP Jason Wang
2020-05-06 6:16 ` [PATCH net-next 2/2] virtio-net: fix the XDP truesize calculation for mergeable buffers Jason Wang
2020-05-06 7:37 ` Michael S. Tsirkin
2020-05-06 8:21 ` Jason Wang
2020-05-06 12:08 ` Michael S. Tsirkin
2020-05-08 1:54 ` Jason Wang
2020-05-06 7:53 ` [PATCH net-next 1/2] virtio-net: don't reserve space for vnet header for XDP Michael S. Tsirkin
2020-05-06 8:19 ` Jason Wang
2020-05-06 9:54 ` Michael S. Tsirkin
2020-05-08 1:56 ` Jason Wang
2020-05-06 8:21 ` Jesper Dangaard Brouer
2020-05-06 8:34 ` Jason Wang [this message]
2020-05-06 9:46 ` Michael S. Tsirkin
2020-05-08 1:59 ` Jason Wang
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