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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 2 Feb 2021 18:02:35 -0500

On Tue, Feb 02, 2021 at 05:17:13PM -0500, Willem de Bruijn wrote:
> On Tue, Feb 2, 2021 at 9:08 AM Michael S. Tsirkin <[email-masked]> wrote:
> >
> > On Mon, Dec 28, 2020 at 11:22:32AM -0500, Willem de Bruijn wrote:
> > > From: Willem de Bruijn <[email-masked]>
> > >
> > > 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 <[email-masked]>
> > > ---
> > >  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?
> 
> I suppose it does not matter as long as this is reserved. Should it be
> __le32, at least?

One can safely assign 0 to any value.

> > > +     __virtio64 tstamp;
> > > +};
> > > +
> >
> > Given it's only available in modern devices, I think we
> > can make this __le64 tstamp.
> 
> Actually, would it be possible to make new features available on
> legacy devices? There is nothing in the features bits precluding it.

I think it won't be possible: you are using feature bit 55,
legacy devices have up to 32 feature bits. And of course the
header looks a bit differently for legacy, you would have to add special
code to handle that when mergeable buffers are off.

> 
> I have a revised patchset almost ready. I suppose I should send it as
> RFC again, and simultaneously file an OASIS ballot for each feature?

that would be great.

-- 
MST