Subject: Re: [PATCH net-next 1/5] virtio: Add support for SCTP checksum offloading

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 16 Apr 2018 20:09:49 +0300

On Mon, Apr 16, 2018 at 09:45:48AM -0400, Vlad Yasevich wrote:
> On 04/11/2018 06:49 PM, Michael S. Tsirkin wrote:
> > On Mon, Apr 02, 2018 at 09:40:02AM -0400, Vladislav Yasevich wrote:
> >> To support SCTP checksum offloading, we need to add a new feature
> >> to virtio_net, so we can negotiate support between the hypervisor
> >> and the guest.
> >>
> >> The signalling to the guest that an alternate checksum needs to
> >> be used is done via a new flag in the virtio_net_hdr.  If the
> >> flag is set, the host will know to perform an alternate checksum
> >> calculation, which right now is only CRC32c.
> >>
> >> Signed-off-by: Vladislav Yasevich <[email-masked]>
> >> ---
> >>  drivers/net/virtio_net.c        | 11 ++++++++---
> >>  include/linux/virtio_net.h      |  6 ++++++
> >>  include/uapi/linux/virtio_net.h |  2 ++
> >>  3 files changed, 16 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >> index 7b187ec..b601294 100644
> >> --- a/drivers/net/virtio_net.c
> >> +++ b/drivers/net/virtio_net.c
> >> @@ -2724,9 +2724,14 @@ static int virtnet_probe(struct virtio_device *vdev)
> >>  	/* Do we support "hardware" checksums? */
> >>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_CSUM)) {
> >>  		/* This opens up the world of extra features. */
> >> -		dev->hw_features |= NETIF_F_HW_CSUM | NETIF_F_SG;
> >> +		netdev_features_t sctp = 0;
> >> +
> >> +		if (virtio_has_feature(vdev, VIRTIO_NET_F_SCTP_CSUM))
> >> +			sctp |= NETIF_F_SCTP_CRC;
> >> +
> >> +		dev->hw_features |= NETIF_F_HW_CSUM | NETIF_F_SG | sctp;
> >>  		if (csum)
> >> -			dev->features |= NETIF_F_HW_CSUM | NETIF_F_SG;
> >> +			dev->features |= NETIF_F_HW_CSUM | NETIF_F_SG | sctp;
> >>  
> >>  		if (virtio_has_feature(vdev, VIRTIO_NET_F_GSO)) {
> >>  			dev->hw_features |= NETIF_F_TSO
> >> @@ -2952,7 +2957,7 @@ static struct virtio_device_id id_table[] = {
> >>  };
> >>  
> >>  #define VIRTNET_FEATURES \
> >> -	VIRTIO_NET_F_CSUM, VIRTIO_NET_F_GUEST_CSUM, \
> >> +	VIRTIO_NET_F_CSUM, VIRTIO_NET_F_GUEST_CSUM,  VIRTIO_NET_F_SCTP_CSUM, \
> >>  	VIRTIO_NET_F_MAC, \
> >>  	VIRTIO_NET_F_HOST_TSO4, VIRTIO_NET_F_HOST_UFO, VIRTIO_NET_F_HOST_TSO6, \
> >>  	VIRTIO_NET_F_HOST_ECN, VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6, \
> >> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> >> index f144216..2e7a64a 100644
> >> --- a/include/linux/virtio_net.h
> >> +++ b/include/linux/virtio_net.h
> >> @@ -39,6 +39,9 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
> >>  
> >>  		if (!skb_partial_csum_set(skb, start, off))
> >>  			return -EINVAL;
> >> +
> >> +		if (hdr->flags & VIRTIO_NET_HDR_F_CSUM_NOT_INET)
> >> +			skb->csum_not_inet = 1;
> >>  	}
> >>  
> >>  	if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {
> >> @@ -96,6 +99,9 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
> >>  		hdr->flags = VIRTIO_NET_HDR_F_DATA_VALID;
> >>  	} /* else everything is zero */
> >>  
> >> +	if (skb->csum_not_inet)
> >> +		hdr->flags &= VIRTIO_NET_HDR_F_CSUM_NOT_INET;
> >> +
> >>  	return 0;
> >>  }
> >>  
> >> diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
> >> index 5de6ed3..3f279c8 100644
> >> --- a/include/uapi/linux/virtio_net.h
> >> +++ b/include/uapi/linux/virtio_net.h
> >> @@ -36,6 +36,7 @@
> >>  #define VIRTIO_NET_F_GUEST_CSUM	1	/* Guest handles pkts w/ partial csum */
> >>  #define VIRTIO_NET_F_CTRL_GUEST_OFFLOADS 2 /* Dynamic offload configuration. */
> >>  #define VIRTIO_NET_F_MTU	3	/* Initial MTU advice */
> >> +#define VIRTIO_NET_F_SCTP_CSUM  4	/* SCTP checksum offload support */
> >>  #define VIRTIO_NET_F_MAC	5	/* Host has given MAC address. */
> >>  #define VIRTIO_NET_F_GUEST_TSO4	7	/* Guest can handle TSOv4 in. */
> >>  #define VIRTIO_NET_F_GUEST_TSO6	8	/* Guest can handle TSOv6 in. */
> > 
> > Is this a guest or a host checksum? We should differenciate between the
> > two.
> 
> I suppose this is HOST checksum, since it behaves like VIRTIO_NET_F_CSUM only for
> SCTP.  I couldn't find the use for the GUEST side flag, since there technically
> isn't any validations and there is no additional mappings from VIRTIO flag to a
> NETIF flag.
> 
> If the feature is negotiated, the guest ends up generating partially check-summed
> packets, and the host turns on appropriate flags on it's side.   The host will
> also make sure the checksum if fixed up if the guest doesn't support it.
> So 1 flag is currently all that is needed.
> 
> -vlad

I see code handling VIRTIO_NET_HDR_F_CSUM_NOT_INET on RX side.  Host
needs to know whether it's ok/worth it to set this flag, too.

> > 
> > 
> >> @@ -101,6 +102,7 @@ struct virtio_net_config {
> >>  struct virtio_net_hdr_v1 {
> >>  #define VIRTIO_NET_HDR_F_NEEDS_CSUM	1	/* Use csum_start, csum_offset */
> >>  #define VIRTIO_NET_HDR_F_DATA_VALID	2	/* Csum is valid */
> >> +#define VIRTIO_NET_HDR_F_CSUM_NOT_INET  4       /* Checksum is not inet */
> >>  	__u8 flags;
> >>  #define VIRTIO_NET_HDR_GSO_NONE		0	/* Not a GSO frame */
> >>  #define VIRTIO_NET_HDR_GSO_TCPV4	1	/* GSO frame, IPv4 TCP (TSO) */
> >> -- 
> >> 2.9.5