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

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Date:   Mon, 28 Dec 2020 14:59:53 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        mst@...hat.com, jasowang@...hat.com,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH rfc 2/3] virtio-net: support receive timestamp

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.