netdev - Re: [PATCH 2/2] virtio_net: fixing XDP for fully checksummed packets handling

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Date: Thu, 20 Jun 2024 16:33:35 +0800
From: Jason Wang <jasowang@...hat.com>
To: Heng Qi <hengqi@...ux.alibaba.com>
Cc: netdev@...r.kernel.org, virtualization@...ts.linux.dev, 
	Thomas Huth <thuth@...ux.vnet.ibm.com>, "Michael S. Tsirkin" <mst@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez <eperezma@...hat.com>, 
	Eric Dumazet <edumazet@...gle.com>, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH 2/2] virtio_net: fixing XDP for fully checksummed packets handling

On Tue, Jun 18, 2024 at 11:17 AM Heng Qi <hengqi@...ux.alibaba.com> wrote:
>
> On Tue, 18 Jun 2024 11:10:26 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > On Mon, Jun 17, 2024 at 9:15 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
> > >
> > > The XDP program can't correctly handle partially checksummed
> > > packets, but works fine with fully checksummed packets.
> >
> > Not sure this is ture, if I was not wrong, XDP can try to calculate checksum.
>
> XDP's interface serves a full checksum,

What do you mean by "serve" here? I mean, XDP can calculate the
checksum and fill it in the packet by itself.

> and this is why we disabled the
> offloading of VIRTIO_NET_F_GUEST_CSUM when loading XDP.

If we trust the device to disable VIRTIO_NET_F_GUEST_CSUM, any reason
to check VIRTIO_NET_HDR_F_NEEDS_CSUM again in the receive path?

>
> Thanks.

Thanks

>
> >
> > Thanks
> >
> > > If the
> > > device has already validated fully checksummed packets, then
> > > the driver doesn't need to re-validate them, saving CPU resources.
> > >
> > > Additionally, the driver does not drop all partially checksummed
> > > packets when VIRTIO_NET_F_GUEST_CSUM is not negotiated. This is
> > > not a bug, as the driver has always done this.
> > >
> > > Fixes: 436c9453a1ac ("virtio-net: keep vnet header zeroed after processing XDP")
> > > Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com>
> > > ---
> > >  drivers/net/virtio_net.c | 20 +++++++++++++++++++-
> > >  1 file changed, 19 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index aa70a7ed8072..ea10db9a09fa 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -1360,6 +1360,10 @@ static struct sk_buff *receive_small_xdp(struct net_device *dev,
> > >         if (unlikely(hdr->hdr.gso_type))
> > >                 goto err_xdp;
> > >
> > > +       /* Partially checksummed packets must be dropped. */
> > > +       if (unlikely(hdr->hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
> > > +               goto err_xdp;
> > > +
> > >         buflen = SKB_DATA_ALIGN(GOOD_PACKET_LEN + headroom) +
> > >                 SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> > >
> > > @@ -1677,6 +1681,10 @@ static void *mergeable_xdp_get_buf(struct virtnet_info *vi,
> > >         if (unlikely(hdr->hdr.gso_type))
> > >                 return NULL;
> > >
> > > +       /* Partially checksummed packets must be dropped. */
> > > +       if (unlikely(hdr->hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
> > > +               return NULL;
> > > +
> > >         /* Now XDP core assumes frag size is PAGE_SIZE, but buffers
> > >          * with headroom may add hole in truesize, which
> > >          * make their length exceed PAGE_SIZE. So we disabled the
> > > @@ -1943,6 +1951,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > >         struct net_device *dev = vi->dev;
> > >         struct sk_buff *skb;
> > >         struct virtio_net_common_hdr *hdr;
> > > +       u8 flags;
> > >
> > >         if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
> > >                 pr_debug("%s: short packet %i\n", dev->name, len);
> > > @@ -1951,6 +1960,15 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > >                 return;
> > >         }
> > >
> > > +       /* 1. Save the flags early, as the XDP program might overwrite them.
> > > +        * These flags ensure packets marked as VIRTIO_NET_HDR_F_DATA_VALID
> > > +        * stay valid after XDP processing.
> > > +        * 2. XDP doesn't work with partially checksummed packets (refer to
> > > +        * virtnet_xdp_set()), so packets marked as
> > > +        * VIRTIO_NET_HDR_F_NEEDS_CSUM get dropped during XDP processing.
> > > +        */
> > > +       flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
> > > +
> > >         if (vi->mergeable_rx_bufs)
> > >                 skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit,
> > >                                         stats);
> > > @@ -1966,7 +1984,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > >         if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
> > >                 virtio_skb_set_hash(&hdr->hash_v1_hdr, skb);
> > >
> > > -       if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
> > > +       if (flags & VIRTIO_NET_HDR_F_DATA_VALID)
> > >                 skb->ip_summed = CHECKSUM_UNNECESSARY;
> > >
> > >         if (virtio_net_hdr_to_skb(skb, &hdr->hdr,
> > > --
> > > 2.32.0.3.g01195cf9f
> > >
> >
>