netdev - Re: [PATCH net-next v2 6/8] gve: Add rx hardware timestamp expansion

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Message-ID: <50be88c9-2cb3-421d-a2bf-4ed9c7d58c58@linux.dev>
Date: Sun, 18 May 2025 22:45:49 +0100
From: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
To: Harshitha Ramamurthy <hramamurthy@...gle.com>, netdev@...r.kernel.org
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, jeroendb@...gle.com, andrew+netdev@...n.ch,
 willemb@...gle.com, ziweixiao@...gle.com, pkaligineedi@...gle.com,
 yyd@...gle.com, joshwash@...gle.com, shailend@...gle.com, linux@...blig.org,
 thostet@...gle.com, jfraker@...gle.com, richardcochran@...il.com,
 jdamato@...tly.com, horms@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2 6/8] gve: Add rx hardware timestamp expansion

On 17.05.2025 01:11, Harshitha Ramamurthy wrote:
> From: John Fraker <jfraker@...gle.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.
> 
> Signed-off-by: John Fraker <jfraker@...gle.com>
> Signed-off-by: Ziwei Xiao <ziweixiao@...gle.com>
> Reviewed-by: Willem de Bruijn <willemb@...gle.com>
> Signed-off-by: Harshitha Ramamurthy <hramamurthy@...gle.com>
> ---
>   Changes in v2:
>   - Add the missing READ_ONCE (Joe Damato)
> ---
>   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..c03c3741e0d4 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 = READ_ONCE(rx->gve->last_sync_nic_counter);

I believe last_read should be u64 as last_sync_nic_counter is u64 and
ns_to_ktime expects u64.

> +	struct sk_buff *skb = rx->ctx.skb_head;
> +	u32 low = (u32)last_read;
> +	s32 diff = hwts - low;
> +
> +	skb_hwtstamps(skb)->hwtstamp = ns_to_ktime(last_read + diff);
> +}
> +
>   static void gve_rx_free_skb(struct napi_struct *napi, struct gve_rx_ring *rx)
>   {
>   	if (!rx->ctx.skb_head)