Subject: Re: [PATCH V2 net-next 1/6] virtio: Add support for SCTP checksum offloading

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 2 May 2018 17:21:38 +0300

On Wed, May 02, 2018 at 11:14:13AM -0300, Marcelo Ricardo Leitner wrote:
> On Wed, May 02, 2018 at 06:16:45AM +0300, Michael S. Tsirkin wrote:
> > On Tue, May 01, 2018 at 10:07:34PM -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 HOST feature bit signifies offloading support for transmit and
> > > enables device offload features.
> > > The GUEST feature bit signifies offloading support of recieve and
> > > is currently only used by the driver in case of xdp.
> > >
> > > That patch also adds an addition virtio_net header flag which
> > > mirrors the skb->csum_not_inet flag.  This flags is used to indicate
> > > that is this an SCTP packet that needs its checksum computed by the
> > > lower layer.  In this case, the lower layer is the host hypervisor or
> > > possibly HW nic that supporst CRC32c offload.
> > >
> > > In the case that GUEST feature bit is flag, it will be possible to
> > > receive a virtio_net header with this bit set, which will set the
> > > corresponding skb bit.  SCTP protocol will be updated to correctly
> > > deal with it.
> > >
> > > Signed-off-by: Vladislav Yasevich <[email-masked]>
> > > ---
> > >  drivers/net/virtio_net.c        | 14 +++++++++++++-
> > >  include/linux/virtio_net.h      |  6 ++++++
> > >  include/uapi/linux/virtio_net.h |  5 +++++
> > >  3 files changed, 24 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 7b187ec..34af280 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -2148,6 +2148,8 @@ static int virtnet_clear_guest_offloads(struct virtnet_info *vi)
> > >
> > >  	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_CSUM))
> > >  		offloads = 1ULL << VIRTIO_NET_F_GUEST_CSUM;
> > > +	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_SCTP_CSUM))
> > > +		offloads |= 1ULL << VIRTIO_NET_F_GUEST_SCTP_CSUM;
> > >
> > >  	return virtnet_set_guest_offloads(vi, offloads);
> > >  }
> > > @@ -2160,6 +2162,8 @@ static int virtnet_restore_guest_offloads(struct virtnet_info *vi)
> > >  		return 0;
> > >  	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_CSUM))
> > >  		offloads |= 1ULL << VIRTIO_NET_F_GUEST_CSUM;
> > > +	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_SCTP_CSUM))
> > > +		offloads |= 1ULL << VIRTIO_NET_F_GUEST_SCTP_CSUM;
> > >
> > >  	return virtnet_set_guest_offloads(vi, offloads);
> > >  }
> > > @@ -2724,6 +2728,7 @@ 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;
> > >  		if (csum)
> > >  			dev->features |= NETIF_F_HW_CSUM | NETIF_F_SG;
> > > @@ -2746,9 +2751,15 @@ static int virtnet_probe(struct virtio_device *vdev)
> > >  			dev->features |= dev->hw_features & NETIF_F_ALL_TSO;
> > >  		/* (!csum && gso) case will be fixed by register_netdev() */
> > >  	}
> > > +
> > >  	if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM))
> > >  		dev->features |= NETIF_F_RXCSUM;
> > >
> > > +	if (virtio_has_feature(vdev, VIRTIO_NET_F_HOST_SCTP_CSUM)) {
> > > +		dev->hw_features |= NETIF_F_SCTP_CRC;
> > > +		dev->features |= NETIF_F_SCTP_CRC;
> > > +	}
> > > +
> > >  	dev->vlan_features = dev->features;
> > >
> > >  	/* MTU range: 68 - 65535 */
> > > @@ -2962,7 +2973,8 @@ static struct virtio_device_id id_table[] = {
> > >  	VIRTIO_NET_F_GUEST_ANNOUNCE, VIRTIO_NET_F_MQ, \
> > >  	VIRTIO_NET_F_CTRL_MAC_ADDR, \
> > >  	VIRTIO_NET_F_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS, \
> > > -	VIRTIO_NET_F_SPEED_DUPLEX
> > > +	VIRTIO_NET_F_SPEED_DUPLEX, \
> > > +	VIRTIO_NET_F_HOST_SCTP_CSUM, VIRTIO_NET_F_GUEST_SCTP_CSUM
> > >
> > >  static unsigned int features[] = {
> > >  	VIRTNET_FEATURES,
> > > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > > index f144216..28fffdc 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) {
> > > @@ -91,6 +94,9 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
> > >  				skb_checksum_start_offset(skb));
> > >  		hdr->csum_offset = __cpu_to_virtio16(little_endian,
> > >  				skb->csum_offset);
> > > +
> > > +		if (skb->csum_not_inet)
> > > +			hdr->flags |= VIRTIO_NET_HDR_F_CSUM_NOT_INET;
> > >  	} else if (has_data_valid &&
> > >  		   skb->ip_summed == CHECKSUM_UNNECESSARY) {
> > >  		hdr->flags = VIRTIO_NET_HDR_F_DATA_VALID;
> > > diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
> > > index 5de6ed3..9dfca1a 100644
> > > --- a/include/uapi/linux/virtio_net.h
> > > +++ b/include/uapi/linux/virtio_net.h
> > > @@ -57,6 +57,10 @@
> > >  					 * Steering */
> > >  #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
> > >
> > > +#define VIRTIO_NET_F_GUEST_SCTP_CSUM  61 /* Guest handles SCTP pks w/ partial
> > > +					  * csum */
> > > +#define VIRTIO_NET_F_HOST_SCTP_CSUM   62 /* HOST handles SCTP pkts w/ partial
> > > +					  * csum */
> > >  #define VIRTIO_NET_F_SPEED_DUPLEX 63	/* Device set linkspeed and duplex */
> > >
> > >  #ifndef VIRTIO_NET_NO_LEGACY
> > > @@ -101,6 +105,7 @@ 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 */
> >
> > Both comment and name are not very informative.
> > How about just saying CRC32c ?
> 
> csum_not_inet is following the nomenclature used in sk_buff. Initially
> Davide had named the sk_buff field after crc32c but then it was argued
> that maybe another check method may be introduced later and the name
> would be bogus, thus why the "csum_not_inet".

That's sk_buff internals, since Linux can change these
at any time without breaking anything.

virtio defines an ABI so we won't be able to make it
mean something else, need to document it fully.
"Go read linux net core code" is not a good answer
to the natural question "what does this bit in the
interface do".

> >
> > >  	__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
> > --
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