Re: [PATCH net-next 4/6] gve: Add rx hardware timestamp expansion

[Joe Damato <jdamato@fastly.com>]

On Fri, Apr 18, 2025 at 10:12:52PM +0000, Harshitha Ramamurthy wrote:
> From: John Fraker <jfraker@google.com>
> 
> Allow the rx path to recover the high 32 bits of the full 64 bit rx
> timestamp.
> 
> Use the low 32 bits of the last synced nic time and the 32 bits of the
> timestamp provided in the rx descriptor to generate a difference, which
> is then applied to the last synced nic time to reconstruct the complete
> 64-bit timestamp.
> 
> This scheme remains accurate as long as no more than ~2 seconds have
> passed between the last read of the nic clock and the timestamping
> application of the received packet.
> 
> Co-developed-by: Ziwei Xiao <ziweixiao@google.com>
> Signed-off-by: Ziwei Xiao <ziweixiao@google.com>
> Reviewed-by: Willem de Bruijn <willemb@google.com>
> Signed-off-by: John Fraker <jfraker@google.com>
> Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
> ---
>  drivers/net/ethernet/google/gve/gve_rx_dqo.c | 23 ++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/drivers/net/ethernet/google/gve/gve_rx_dqo.c b/drivers/net/ethernet/google/gve/gve_rx_dqo.c
> index dcb0545baa50..483d188d33ab 100644
> --- a/drivers/net/ethernet/google/gve/gve_rx_dqo.c
> +++ b/drivers/net/ethernet/google/gve/gve_rx_dqo.c
> @@ -437,6 +437,29 @@ static void gve_rx_skb_hash(struct sk_buff *skb,
>  	skb_set_hash(skb, le32_to_cpu(compl_desc->hash), hash_type);
>  }
>  
> +/* Expand the hardware timestamp to the full 64 bits of width, and add it to the
> + * skb.
> + *
> + * This algorithm works by using the passed hardware timestamp to generate a
> + * diff relative to the last read of the nic clock. This diff can be positive or
> + * negative, as it is possible that we have read the clock more recently than
> + * the hardware has received this packet. To detect this, we use the high bit of
> + * the diff, and assume that the read is more recent if the high bit is set. In
> + * this case we invert the process.
> + *
> + * Note that this means if the time delta between packet reception and the last
> + * clock read is greater than ~2 seconds, this will provide invalid results.
> + */
> +static void __maybe_unused gve_rx_skb_hwtstamp(struct gve_rx_ring *rx, u32 hwts)
> +{
> +	s64 last_read = rx->gve->last_sync_nic_counter;

Does this need a READ_ONCE to match the WRITE_ONCE in the previous
patch ?