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Date:   Wed, 30 Dec 2020 16:44:17 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Richard Cochran <richardcochran@...il.com>
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 <kuba@...nel.org> wrote:
>> On Mon, 28 Dec 2020 11:22:32 -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.
>> 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.

Thanks


>

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