Re: [PATCH net-next v8 6/7] net: bcmgenet: add XDP statistics counters

[Nicolai Buchwitz <nb@tipi-net.de>]

On 29.4.2026 23:17, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 3 potential 
> issue(s) to consider:
> - [High] XDP_TX headroom check ignores metadata size, and TSB insertion 
> corrupts metadata state.
> - [Medium] Non-atomic updates to global XDP statistics from concurrent 
> NAPI contexts cause data races and cache-line bouncing.
> - [Medium] Missing counter increment when `xdp_convert_buff_to_frame()` 
> fails in the `XDP_TX` path.
> --
> 
> commit 9febde2d94a02157dc480d909cc304c59196d72f
> Author: Nicolai Buchwitz <nb@tipi-net.de>
> 
> net: bcmgenet: add XDP statistics counters
> 
> This commit exposes per-action XDP counters via ethtool -S. It uses
> the soft MIB infrastructure and increments the counters in
> bcmgenet_run_xdp().
> 
>> diff --git a/drivers/net/ethernet/broadcom/genet/bcmgenet.c 
>> b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>> index 9dd258567824..02ad2f410d6c 100644
>> --- a/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>> +++ b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
> 
> [ ... ]
> 
>> @@ -2428,6 +2435,7 @@ static unsigned int bcmgenet_run_xdp(struct 
>> bcmgenet_rx_ring *ring,
>> 
>>  	switch (act) {
>>  	case XDP_PASS:
>> +		priv->mib.xdp_pass++;
> 
> Is it safe to update global priv->mib statistics here?

The XDP counters follow the existing soft MIB convention in this
driver (plain u32, unsynchronized). A larger ring refactoring is
AFAIK planned that should address all MIB counters consistently.

> 
> Since bcmgenet_run_xdp() can execute concurrently in multiple 
> independent
> NAPI polling contexts (one per RX ring), non-atomic increments to 
> shared
> global variables can cause data races and lost increments.
> 
> Continually modifying a shared cacheline on the fast-path for every 
> packet
> could also force cacheline bouncing across CPUs and degrade XDP 
> performance.
> Should these statistics be maintained per-queue or using per-CPU 
> variables
> instead?
> 
>>  		return XDP_PASS;
>>  	case XDP_TX:
>>  		/* Prepend a zeroed TSB (Transmit Status Block).  The GENET
>>  		 * MAC has TBUF_64B_EN set globally, so hardware expects every
>>  		 * TX buffer to begin with a 64-byte struct status_64.  Back
>>  		 * up xdp->data into the RSB area (which is no longer needed
>>  		 * after the BPF program ran) and zero it.
>>  		 */
>>  		if (xdp->data - xdp->data_hard_start <
>>  		    sizeof(struct status_64) + sizeof(struct xdp_frame)) {
>>  			page_pool_put_full_page(ring->page_pool, rx_page,
>>  						true);
>> +			priv->mib.xdp_tx_err++;
>>  			return XDP_DROP;
>>  		}
>>  		xdp->data -= sizeof(struct status_64);
>>  		xdp->data_meta -= sizeof(struct status_64);
>>  		memset(xdp->data, 0, sizeof(struct status_64));
> 
> While not introduced by this patch, does this headroom check and TSB
> insertion handle metadata correctly?

Already addressed under patches 4 and 5: xdp_convert_buff_to_frame()
validates headroom against metasize. The memset writes into the former
RSB area, above the metadata; both data and data_meta shift equally so
the metadata position is preserved.

> 
> The headroom check doesn't account for metasize, which could cause
> xdp_convert_buff_to_frame() to fail its own bounds check later.
> 
> Additionally, if the XDP program added metadata, it would be located at
> [old_data - metasize, old_data]. The memset writes zeroes to
> [new_data, new_data + 64], which is [old_data - 64, old_data], 
> potentially
> overwriting the original metadata.
> 
> Since xdp->data_meta is shifted back by 64 bytes, could it now point to
> garbage memory and corrupt the metadata state?
> 
>> 
>>  		xdpf = xdp_convert_buff_to_frame(xdp);
>>  		if (unlikely(!xdpf)) {
>>  			page_pool_put_full_page(ring->page_pool, rx_page,
>>  						true);
>>  			return XDP_DROP;
>>  		}
> 
> Are we missing a counter increment on this failure path?

Fair point, but xdp_convert_buff_to_frame() failure is an extremely
rare path (insufficient headroom for xdp_frame). Can be addressed in
a follow-up.

> 
> The preceding headroom check failure explicitly increments xdp_tx_err 
> before
> dropping, but this path returns directly. Since it doesn't fall through 
> to
> the XDP_DROP switch case, the dropped packet appears to be unaccounted 
> for
> in the new statistics.

Thanks,
Nicolai