netdev - Re: [PATCH RFC v2 3/4] virtio-net: support transmit timestamp

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Message-ID: <CAF=yD-LepCKmpRbQyHP5+61ewCSpwOHmsQm_GNBcTi03Z5knXA@mail.gmail.com>
Date:   Tue, 9 Feb 2021 09:45:08 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>,
        Jason Wang <jasowang@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH RFC v2 3/4] virtio-net: support transmit timestamp

On Tue, Feb 9, 2021 at 4:43 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Mon, Feb 08, 2021 at 01:55:57PM -0500, Willem de Bruijn wrote:
> > From: Willem de Bruijn <willemb@...gle.com>
> >
> > Add optional PTP hardware tx timestamp offload for virtio-net.
> >
> > Accurate RTT measurement requires timestamps close to the wire.
> > Introduce virtio feature VIRTIO_NET_F_TX_TSTAMP, the transmit
> > equivalent to VIRTIO_NET_F_RX_TSTAMP.
> >
> > The driver sets VIRTIO_NET_HDR_F_TSTAMP to request a timestamp
> > returned on completion. If the feature is negotiated, the device
> > either places the timestamp or clears the feature bit.
> >
> > The timestamp straddles (virtual) hardware domains. Like PTP, use
> > international atomic time (CLOCK_TAI) as global clock base. The driver
> > must sync with the device, e.g., through kvm-clock.
> >
> > Modify can_push to ensure that on tx completion the header, and thus
> > timestamp, is in a predicatable location at skb_vnet_hdr.
> >
> > RFC: this implementation relies on the device writing to the buffer.
> > That breaks DMA_TO_DEVICE semantics. For now, disable when DMA is on.
>
> If you do something like this, please do it in the validate
> callback and clear the features you aren't using.

Ah yes. Thanks for the tip. I'll do that ..

.. once I'm sure that this approach of using an outbuf for I/O is
actually allowed behavior. I'm not entirely convinced yet myself.
Jason also pointed out more specific concerns. I'll look into that
further.