Re: [PATCH RFC v2 3/4] virtio-net: support transmit timestamp
From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: 2021-02-09 14:46:48
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netdev
On Tue, Feb 9, 2021 at 4:43 AM Michael S. Tsirkin [off-list ref] wrote:
On Mon, Feb 08, 2021 at 01:55:57PM -0500, Willem de Bruijn wrote:quoted
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