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From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: virtualization@lists.linux-foundation.org,
	Network Development <netdev@vger.kernel.org>,
	Jason Wang <jasowang@redhat.com>,
	Richard Cochran <richardcochran@gmail.com>,
	Willem de Bruijn <willemb@google.com>
Subject: Re: [PATCH RFC v2 3/4] virtio-net: support transmit timestamp
Date: Tue, 9 Feb 2021 09:45:08 -0500	[thread overview]
Message-ID: <CAF=yD-LepCKmpRbQyHP5+61ewCSpwOHmsQm_GNBcTi03Z5knXA@mail.gmail.com> (raw)
In-Reply-To: <20210209044125-mutt-send-email-mst@kernel.org>

On Tue, Feb 9, 2021 at 4:43 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Mon, Feb 08, 2021 at 01:55:57PM -0500, Willem de Bruijn wrote:
> > From: Willem de Bruijn <willemb@google.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.

WARNING: multiple messages have this Message-ID (diff)
From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Willem de Bruijn <willemb@google.com>,
	Network Development <netdev@vger.kernel.org>,
	Richard Cochran <richardcochran@gmail.com>,
	virtualization@lists.linux-foundation.org
Subject: Re: [PATCH RFC v2 3/4] virtio-net: support transmit timestamp
Date: Tue, 9 Feb 2021 09:45:08 -0500	[thread overview]
Message-ID: <CAF=yD-LepCKmpRbQyHP5+61ewCSpwOHmsQm_GNBcTi03Z5knXA@mail.gmail.com> (raw)
In-Reply-To: <20210209044125-mutt-send-email-mst@kernel.org>

On Tue, Feb 9, 2021 at 4:43 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Mon, Feb 08, 2021 at 01:55:57PM -0500, Willem de Bruijn wrote:
> > From: Willem de Bruijn <willemb@google.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.
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

  reply	other threads:[~2021-02-09 14:46 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-08 18:55 [PATCH RFC v2 0/4] virtio-net: add tx-hash, rx-tstamp, tx-tstamp and tx-time Willem de Bruijn
2021-02-08 18:55 ` Willem de Bruijn
2021-02-08 18:55 ` [PATCH RFC v2 1/4] virtio-net: support transmit hash report Willem de Bruijn
2021-02-08 18:55   ` Willem de Bruijn
2021-02-08 18:55 ` [PATCH RFC v2 2/4] virtio-net: support receive timestamp Willem de Bruijn
2021-02-08 18:55   ` Willem de Bruijn
2021-02-09  4:13   ` Jason Wang
2021-02-09 13:53     ` Willem de Bruijn
2021-02-09 13:53       ` Willem de Bruijn
2021-02-10  4:10       ` Jason Wang
2021-02-10  4:10         ` Jason Wang
2021-02-08 18:55 ` [PATCH RFC v2 3/4] virtio-net: support transmit timestamp Willem de Bruijn
2021-02-08 18:55   ` Willem de Bruijn
2021-02-09  5:45   ` Jason Wang
2021-02-09  5:45     ` Jason Wang
2021-02-09 16:38     ` Michael S. Tsirkin
2021-02-09 16:38       ` Michael S. Tsirkin
2021-02-10  2:36       ` Willem de Bruijn
2021-02-10  2:36         ` Willem de Bruijn
2021-02-10  4:15         ` Jason Wang
2021-02-10  4:15           ` Jason Wang
2021-02-10 14:40           ` Willem de Bruijn
2021-02-10 14:40             ` Willem de Bruijn
2021-02-10  4:12       ` Jason Wang
2021-02-10  4:12         ` Jason Wang
2021-02-09  9:42   ` Michael S. Tsirkin
2021-02-09 14:45     ` Willem de Bruijn [this message]
2021-02-09 14:45       ` Willem de Bruijn
2021-02-08 18:55 ` [PATCH RFC v2 4/4] virtio-net: support future packet transmit time Willem de Bruijn
2021-02-08 18:55   ` Willem de Bruijn
2021-05-13 22:49 ` [PATCH RFC v2 0/4] virtio-net: add tx-hash, rx-tstamp, tx-tstamp and tx-time Willem de Bruijn
2021-05-14  7:12   ` Jason Wang
2021-05-14  7:12     ` Jason Wang
2021-05-14 12:46     ` Willem de Bruijn
2021-05-14 12:46       ` Willem de Bruijn

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