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Date:   Wed, 30 Dec 2020 16:44:17 +0800
From:   Jason Wang <>
To:     Willem de Bruijn <>,
        Jakub Kicinski <>
        Network Development <>,
        "Michael S. Tsirkin" <>,
        Richard Cochran <>
Subject: Re: [PATCH rfc 2/3] virtio-net: support receive timestamp

On 2020/12/29 上午8:57, Willem de Bruijn wrote:
> On Mon, Dec 28, 2020 at 5:59 PM Jakub Kicinski <> wrote:
>> On Mon, 28 Dec 2020 11:22:32 -0500 Willem de Bruijn wrote:
>>> From: Willem de Bruijn <>
>>> 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.
>> Would this not be confusing to some user space SW to have a NIC with
>> no PHC deliver HW stamps?
>> I'd CC Richard on this, unless you already discussed with him offline.
> Thanks, good point. I should have included Richard.
> There is a well understood method for synchronizing guest and host
> clock in KVM using ptp_kvm. For virtual environments without NIC
> hardware offload, the when host timestamps in software, this suffices.
> Syncing host with NIC is assumed if the host advertises the feature
> and implements using real hardware timestamps.

Or it could be useful for virtio hardware when there's no KVM that 
provides PTP.



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