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

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Message-ID: <CA+FuTSffjFZGGwpMkpnTBbHHwnJN7if=0cVf0Des7Db5UFe0bw@mail.gmail.com>
Date:   Mon, 28 Dec 2020 20:05:29 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH rfc 2/3] virtio-net: support receive timestamp

On Mon, Dec 28, 2020 at 7:55 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Mon, Dec 28, 2020 at 02:30:31PM -0500, Willem de Bruijn wrote:
> > On Mon, Dec 28, 2020 at 12:29 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > >
> > > On Mon, Dec 28, 2020 at 11:22:32AM -0500, Willem de Bruijn wrote:
> > > > From: Willem de Bruijn <willemb@...gle.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@...gle.com>
> > > > 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;
> > > > +     __virtio64 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
> > >
> > >
> > > So it looks like VIRTIO_NET_F_RX_TSTAMP should depend on both
> > > VIRTIO_NET_F_RX_TSTAMP and VIRTIO_NET_F_HASH_REPORT then?
> >
> > Do you mean VIRTIO_NET_F_TX_TSTAMP depends on VIRTIO_NET_F_RX_TSTAMP?
> >
> > I think if either is enabled we need to enable the extended layout.
> > Regardless of whether TX_HASH or HASH_REPORT are enabled. If they are
> > not, then those fields are ignored.
>
> I mean do we waste the 8 bytes for hash if TSTAMP is set but HASH is not?
> If TSTAMP depends on HASH then point is moot.

True, but the two features really are independent.

I did consider using configurable metadata layout depending on
features negotiated. If there are tons of optional extensions, that
makes sense. But it is more complex and parsing error prone. With a
handful of options each of a few bytes, that did not seem worth the
cost to me at this point.

And importantly, such a mode can always be added later as a separate
VIRTIO_NET_F_PACKED_HEADER option.

If anything, perhaps if we increase the virtio_net_hdr_* allocation,
we should allocate some additional reserved space now? As each new
size introduces quite a bit of boilerplate. Also, e.g., in qemu just
to pass the sizes between virtio-net driver and vhost-net device.