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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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