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Message-ID: <20240620061710-mutt-send-email-mst@kernel.org>
Date: Thu, 20 Jun 2024 06:19:01 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Heng Qi <hengqi@...ux.alibaba.com>
Cc: Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
virtualization@...ts.linux.dev,
Thomas Huth <thuth@...ux.vnet.ibm.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 Thu, Jun 20, 2024 at 05:28:48PM +0800, Heng Qi wrote:
> On Thu, 20 Jun 2024 16:33:35 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > 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.
> >
>
> Yes, XDP can parse and calculate checksums for all packets.
> However, the bpf_csum_diff and bpf_l4_csum_replace APIs provided by XDP assume
> that the packets being processed are fully checksumed packets. That is,
> after the XDP program modified the packets, the incremental checksum can be
> calculated (for example, samples/bpf/tcbpf1_kern.c, samples/bpf/test_lwt_bpf.c).
>
> Therefore, partially checksummed packets cannot be processed normally in these
> examples and need to be discarded.
>
> > > 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?
>
> There doesn't seem to be a mandatory constraint in the spec that devices that
> haven't negotiated VIRTIO_NET_F_GUEST_CSUM cannot set NEEDS_CSUM bit, so I check this.
>
> Thanks.
The spec says:
\item If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} can be
set: if so, the packet checksum at offset \field{csum_offset}
from \field{csum_start} and any preceding checksums
have been validated. The checksum on the packet is incomplete and
if bit VIRTIO_NET_HDR_F_RSC_INFO is not set in \field{flags},
then \field{csum_start} and \field{csum_offset} indicate how to calculate it
(see Packet Transmission point 1).
So yes, NEEDS_CSUM without VIRTIO_NET_F_GUEST_CSUM is at best undefined.
Please do not try to use it unless VIRTIO_NET_F_GUEST_CSUM is set.
And if you want to be flexible, ignore it unless VIRTIO_NET_F_GUEST_CSUM
has been negotiated.
> >
> > >
> > > 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
> > > > >
> > > >
> > >
> >
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