On Mon, Jun 26, 2023 at 08:14:04AM -0400, Michael S. Tsirkin wrote:
> On Mon, Jun 26, 2023 at 08:03:00PM +0800, Heng Qi wrote:
> > We are now re-probing the csum related fields and trying
> > to have XDP and RX hw checksum capabilities coexist on the
> > XDP path. For the benefit of:
> > 1. RX hw checksum capability can be used if XDP is loaded.
> > 2. Avoid packet loss when loading XDP in the vm-vm scenario.
> >
> > Signed-off-by: Heng Qi <hengqi@xxxxxxxxxxxxxxxxx>
> > Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> > ---
> > v2->v3:
> > - Use skb_checksum_setup() instead of virtnet_flow_dissect_udp_tcp().
> > Essentially equivalent.
> >
> > drivers/net/virtio_net.c | 86 ++++++++++++++++++++++++++++++++++------
> > 1 file changed, 73 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 5a7f7a76b920..0a715e0fbc97 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1568,6 +1568,44 @@ static void virtio_skb_set_hash(const struct virtio_net_hdr_v1_hash *hdr_hash,
> > skb_set_hash(skb, __le32_to_cpu(hdr_hash->hash_value), rss_hash_type);
> > }
> >
> > +static int virtnet_set_csum_after_xdp(struct virtnet_info *vi,
> > + struct sk_buff *skb,
> > + __u8 flags)
> > +{
> > + int err = 0;
> > +
> > + /* When XDP program is loaded, for example, the vm-vm scenario
> > + * on the same host, packets marked as VIRTIO_NET_HDR_F_NEEDS_CSUM
> > + * will travel. Although these packets are safe from the point of
> > + * view of the vm, to avoid modification by XDP and successful
> > + * forwarding in the upper layer,
>
> why do you want tp avoid forwarding? did you mean
> "and to allow forwarding"?
>
Yes, I mean "to allow forwarding".
> > we re-probe the necessary checksum
> > + * related information: skb->csum_{start, offset}, pseudo-header csum
> > + * using skb_chdcksum_setup().
>
> typo
>
Will fix.
> > + *
> > + * This benefits us:
>
> Drop "This benefits us:" - benefits compared to what?
Benefits compared to not having this patch.
>
> > + * 1. XDP can be loaded when there's _F_GUEST_CSUM.
> > + * 2. The device verifies the checksum of packets, especially
> > + * benefiting for large packets.
> > + * 3. In the same-host vm-vm scenario, packets marked as
> > + * VIRTIO_NET_HDR_F_NEEDS_CSUM are no longer dropped after being
> > + * processed by XDP.
>
> please rewrite so the text makes sense in the final C file,
> not for someone reading the diff. In that cotext it does not
> matter that we used to drop packets or that we used to
> disable _F_GUEST_CSUM unless you explain
> why they had to be dropped previously.
Reasonable, will be rewritten from the reader's point of view.
>
>
> > + */
> > + if (flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
> > + /* We don't parse SCTP because virtio-net currently doesn't
> > + * support CRC checksum offloading for SCTP.
>
> what does this refer to?
It means that the SCTP packets received by the virtio-net rx side have
complete checksums instead of only partial checksums (pseudo-header
checksums). Because virtio-net does not provide the NETIF_F_SCTP_CRC feature.
> where does it exclude SCTP?
Please see:
--> skb_checksum_setup()
--> skb_checksum_setup_ipv4()
--> skb_checksum_setup_ip()
--> IPPROTO_TCP/IPPROTO_UDP
--> skb_checksum_setup_ipv6()
--> skb_checksum_setup_ip()
--> IPPROTO_TCP/IPPROTO_UDP
>
> > + */
> > + err = skb_checksum_setup(skb, true);
> > + } else if (flags & VIRTIO_NET_HDR_F_DATA_VALID) {
> > + /* We want to benefit from this: XDP guarantees that packets marked
> > + * as VIRTIO_NET_HDR_F_DATA_VALID still have correct csum after they
> > + * are processed.
>
> drop "We want to benefit from this: "
Ok.
>
> > + */
> > + skb->ip_summed = CHECKSUM_UNNECESSARY;
> > + }
> > +
> > + return err;
> > +}
> > +
> > static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > void *buf, unsigned int len, void **ctx,
> > unsigned int *xdp_xmit,
> > @@ -1576,6 +1614,7 @@ 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_hdr_mrg_rxbuf *hdr;
> > + __u8 flags;
> >
> > if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
> > pr_debug("%s: short packet %i\n", dev->name, len);
> > @@ -1584,6 +1623,13 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > return;
> > }
> >
> > + /* Save the flags of the hdr before XDP processes the data.
> > + * It is ok to use this for both mergeable and small modes.
> > + * Because that's what we do now.
>
> What does the last sentence mean?
It means that both mergeable and small modes can use the
virtio_net_hdr_mrg_rxbuf* structure to obtain the flags in it, to remind
readers that this sentence works. Because flags are located at the same
position in the virtio_net_hdr_mrg_rxbuf and virtio_net_hdr structures.
Example of existing code:
--> receive_small_xdp():
struct virtio_net_hdr_mrg_rxbuf *hdr = buf + header_offset;
> Instead please explain why this is necessary. what can change the
> header?
Loaded XDP may overwrite/modify headers.
>
> > + */
> > + if (unlikely(vi->xdp_enabled))
> > + flags = ((struct virtio_net_hdr_mrg_rxbuf *)buf)->hdr.flags;
> > +
> > if (vi->mergeable_rx_bufs)
> > skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit,
> > stats);
> > @@ -1595,23 +1641,37 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > if (unlikely(!skb))
> > return;
> >
> > - hdr = skb_vnet_hdr(skb);
> > - if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
> > - virtio_skb_set_hash((const struct virtio_net_hdr_v1_hash *)hdr, skb);
> > -
> > - if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
> > - skb->ip_summed = CHECKSUM_UNNECESSARY;
> > + if (unlikely(vi->xdp_enabled)) {
> > + /* Required to do this before re-probing and calculating
> > + * the pseudo-header checksum.
>
> What if checksum was disabled on device?
When XDP is loaded, it may modify or overwrite the packet information.
If the packet has a partial checksum, virtio-net needs to recalculate
the partial checksum, it doesn't matter if the device has F_GUEST_CSUM
or not.
If the packet has a complete checksum and the device does not have
F_GUEST_CSUM, virtio-net does nothing and it is up to the stack to
verify the checksum.
>No need for all the elaborate hacks then, right?
No, always required when xdp is loaded.
> What about disabling by ethtool?
No way. guest csum is a [fixed] feature.
Thanks.
>
>
> > + */
> > + skb->protocol = eth_type_trans(skb, dev);
> > + skb_reset_network_header(skb);
> > + if (virtnet_set_csum_after_xdp(vi, skb, flags) < 0) {
> > + pr_debug("%s: errors occurred in setting partial csum",
> > + dev->name);
> > + goto frame_err;
> > + }
> > + } else {
> > + hdr = skb_vnet_hdr(skb);
> > + if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
> > + virtio_skb_set_hash((const struct virtio_net_hdr_v1_hash *)hdr, skb);
> > +
> > + if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
> > + skb->ip_summed = CHECKSUM_UNNECESSARY;
> > +
> > + if (virtio_net_hdr_to_skb(skb, &hdr->hdr,
> > + virtio_is_little_endian(vi->vdev))) {
> > + net_warn_ratelimited("%s: bad gso: type: %u, size: %u\n",
> > + dev->name, hdr->hdr.gso_type,
> > + hdr->hdr.gso_size);
> > + goto frame_err;
> > + }
> >
> > - if (virtio_net_hdr_to_skb(skb, &hdr->hdr,
> > - virtio_is_little_endian(vi->vdev))) {
> > - net_warn_ratelimited("%s: bad gso: type: %u, size: %u\n",
> > - dev->name, hdr->hdr.gso_type,
> > - hdr->hdr.gso_size);
> > - goto frame_err;
> > + skb->protocol = eth_type_trans(skb, dev);
> > }
> >
> > skb_record_rx_queue(skb, vq2rxq(rq->vq));
> > - skb->protocol = eth_type_trans(skb, dev);
> > pr_debug("Receiving skb proto 0x%04x len %i type %i\n",
> > ntohs(skb->protocol), skb->len, skb->pkt_type);
> >
> > --
> > 2.19.1.6.gb485710b
