On Thu, Jun 25, 2026 at 09:36:18AM +0000, Ciara Loftus wrote:
> The number of Rx packets is computed as the sum of the unicast,
> multicast and broadcast packet counters, minus the discarded packet
> count:
>
> ipackets = rx_unicast + rx_multicast + rx_broadcast - rx_discards
>
> The unicast, multicast and broadcast counters already include the
> packets that were subsequently dropped, so subtracting rx_discards
> yields only the packets delivered to the application. These values are
> provided by the PF in a virtchnl_eth_stats message; the PF samples them
> from separate sources and the order in which they are read cannot be
> guaranteed. Under load, rx_discards can therefore momentarily exceed the
> sum of the unicast, multicast and broadcast counters. As ipackets is
> unsigned, the subtraction then wraps to a huge bogus value, reported to
> the application as an enormous Rx packet count and packet rate.
>
> The read order cannot be guaranteed, so use a saturating subtraction:
> when rx_discards exceeds the sum of the unicast, multicast and broadcast
> counters essentially nothing was delivered, so report zero instead of
> underflowing.
>
> Fixes: c9f889e99616 ("net/ice: enable stats for DCF")
> Cc: [email protected]
>
> Signed-off-by: Ciara Loftus <[email protected]>
> ---
Acked-by: Bruce Richardson <[email protected]>
