Re: [PATCH 0/3] net/intel: fix potential rx stats underflow

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, Jun 25, 2026 at 09:36:16AM +0000, Ciara Loftus wrote:
> The Rx packet count reported by ice, ice DCF and iavf can momentarily
> jump to an enormous invalid value under load. ipackets is derived by
> subtracting the discarded packet count from the sum of the unicast,
> multicast and broadcast counters. That sum already includes the
> discarded packets, so the result is the number actually delivered. The
> inputs are sampled from separate sources at slightly different instants,
> so the discard count can briefly exceed the measured sum. Because the
> arithmetic is unsigned, the subtraction wraps to a huge value and is
> reported as a hugely incorrect packet count and rate.
> 
> All three share the bug but not the fix, because they differ in how
> much control they have over the sampling.
> 
> The ice PF reads its counters directly from hardware registers, so the
> order is under the driver's control. Reading the discard register before
> the other three, combined with the fact that the counters only ever
> increase and the delivered-packet sum always includes the discards,
> makes it impossible for the discard count to exceed the later sum. No
> clamping is needed and the subtraction can never underflow.
> 
> ice DCF and iavf receive their counters from the PF in a single virtchnl
> message and cannot influence the order in which the PF sampled them.
> There a reorder is not available, so the subtraction is made saturating:
> when the discard count exceeds the sum, essentially nothing was delivered,
> so zero is reported instead of underflowing.
> 
> Ciara Loftus (3):
>   net/ice: fix Rx packets statistics underflow
>   net/ice: fix DCF Rx packets statistics underflow
>   net/iavf: fix Rx packets statistics underflow
> 
Series applied to dpdk-next-net-intel. I was tempted to squash into one
patch, but kept them as three because the fix is not identical in all cases
(iavf being different).

Thanks,
/Bruce