Re: [PATCH rfc 2/3] virtio-net: support receive timestamp

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Dec 28, 2020 at 11:22:32AM -0500, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.com>
> 
> Add optional PTP hardware timestamp offload for virtio-net.
> 
> Accurate RTT measurement requires timestamps close to the wire.
> Introduce virtio feature VIRTIO_NET_F_RX_TSTAMP. If negotiated, the
> virtio-net header is expanded with room for a timestamp. A host may
> pass receive timestamps for all or some packets. A timestamp is valid
> if non-zero.
> 
> The timestamp straddles (virtual) hardware domains. Like PTP, use
> international atomic time (CLOCK_TAI) as global clock base. It is
> guest responsibility to sync with host, e.g., through kvm-clock.
> 
> Signed-off-by: Willem de Bruijn <willemb@google.com>
> ---
>  drivers/net/virtio_net.c        | 20 +++++++++++++++++++-
>  include/uapi/linux/virtio_net.h | 12 ++++++++++++
>  2 files changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index b917b7333928..57744bb6a141 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -204,6 +204,9 @@ struct virtnet_info {
>  	/* Guest will pass tx path info to the host */
>  	bool has_tx_hash;
>  
> +	/* Host will pass CLOCK_TAI receive time to the guest */
> +	bool has_rx_tstamp;
> +
>  	/* Has control virtqueue */
>  	bool has_cvq;
>  
> @@ -292,6 +295,13 @@ static inline struct virtio_net_hdr_mrg_rxbuf *skb_vnet_hdr(struct sk_buff *skb)
>  	return (struct virtio_net_hdr_mrg_rxbuf *)skb->cb;
>  }
>  
> +static inline struct virtio_net_hdr_v12 *skb_vnet_hdr_12(struct sk_buff *skb)
> +{
> +	BUILD_BUG_ON(sizeof(struct virtio_net_hdr_v12) > sizeof(skb->cb));
> +
> +	return (void *)skb->cb;
> +}
> +
>  /*
>   * private is used to chain pages for big packets, put the whole
>   * most recent used list in the beginning for reuse
> @@ -1082,6 +1092,9 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>  		goto frame_err;
>  	}
>  
> +	if (vi->has_rx_tstamp)
> +		skb_hwtstamps(skb)->hwtstamp = ns_to_ktime(skb_vnet_hdr_12(skb)->tstamp);
> +
>  	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",
> @@ -3071,6 +3084,11 @@ static int virtnet_probe(struct virtio_device *vdev)
>  		vi->hdr_len = sizeof(struct virtio_net_hdr_v1_hash);
>  	}
>  
> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_RX_TSTAMP)) {
> +		vi->has_rx_tstamp = true;
> +		vi->hdr_len = sizeof(struct virtio_net_hdr_v12);
> +	}
> +
>  	if (virtio_has_feature(vdev, VIRTIO_F_ANY_LAYOUT) ||
>  	    virtio_has_feature(vdev, VIRTIO_F_VERSION_1))
>  		vi->any_header_sg = true;
> @@ -3261,7 +3279,7 @@ static struct virtio_device_id id_table[] = {
>  	VIRTIO_NET_F_CTRL_MAC_ADDR, \
>  	VIRTIO_NET_F_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS, \
>  	VIRTIO_NET_F_SPEED_DUPLEX, VIRTIO_NET_F_STANDBY, \
> -	VIRTIO_NET_F_TX_HASH
> +	VIRTIO_NET_F_TX_HASH, VIRTIO_NET_F_RX_TSTAMP
>  
>  static unsigned int features[] = {
>  	VIRTNET_FEATURES,
> diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
> index f6881b5b77ee..0ffe2eeebd4a 100644
> --- a/include/uapi/linux/virtio_net.h
> +++ b/include/uapi/linux/virtio_net.h
> @@ -57,6 +57,7 @@
>  					 * Steering */
>  #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
>  
> +#define VIRTIO_NET_F_RX_TSTAMP	  55	/* Host sends TAI receive time */
>  #define VIRTIO_NET_F_TX_HASH	  56	/* Guest sends hash report */
>  #define VIRTIO_NET_F_HASH_REPORT  57	/* Supports hash report */
>  #define VIRTIO_NET_F_RSS	  60	/* Supports RSS RX steering */
> @@ -182,6 +183,17 @@ struct virtio_net_hdr_v1_hash {
>  	};
>  };
>  
> +struct virtio_net_hdr_v12 {
> +	struct virtio_net_hdr_v1 hdr;
> +	struct {
> +		__le32 value;
> +		__le16 report;
> +		__le16 flow_state;
> +	} hash;
> +	__virtio32 reserved;


Does endian-ness matter? If not - just u32?

> +	__virtio64 tstamp;
> +};
> +

Given it's only available in modern devices, I think we
can make this __le64 tstamp.

>  #ifndef VIRTIO_NET_NO_LEGACY
>  /* This header comes first in the scatter-gather list.
>   * For legacy virtio, if VIRTIO_F_ANY_LAYOUT is not negotiated, it must
> -- 
> 2.29.2.729.g45daf8777d-goog

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