> -----Original Message-----
> From: Intel-wired-lan <intel-wired-lan-bounces@xxxxxxxxxx> On Behalf
> Of Jason Xing
> Sent: Tuesday, July 22, 2025 3:51 PM
> To: Nguyen, Anthony L <anthony.l.nguyen@xxxxxxxxx>; Kitszel,
> Przemyslaw <przemyslaw.kitszel@xxxxxxxxx>; andrew+netdev@xxxxxxx;
> davem@xxxxxxxxxxxxx; edumazet@xxxxxxxxxx; kuba@xxxxxxxxxx;
> pabeni@xxxxxxxxxx; bjorn@xxxxxxxxxx; Karlsson, Magnus
> <magnus.karlsson@xxxxxxxxx>; Fijalkowski, Maciej
> <maciej.fijalkowski@xxxxxxxxx>; jonathan.lemon@xxxxxxxxx;
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> alexandre.torgue@xxxxxxxxxxx
> Cc: linux-stm32@xxxxxxxxxxxxxxxxxxxxxxxxxxxx; bpf@xxxxxxxxxxxxxxx;
> intel-wired-lan@xxxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Jason Xing
> <kernelxing@xxxxxxxxxxx>
> Subject: [Intel-wired-lan] [PATCH net v2 2/2] igb: xsk: solve
> underflow of nb_pkts in zerocopy mode
>
> From: Jason Xing <kernelxing@xxxxxxxxxxx>
>
> There is no break time in the while() loop, so every time at the end
> of igb_xmit_zc(), underflow of nb_pkts will occur, which renders the
> return value always false. But theoretically, the result should be set
> after calling xsk_tx_peek_release_desc_batch(). We can take
> i40e_xmit_zc() as a good example.
>
> Returning false means we're not done with transmission and we need one
> more poll, which is exactly what igb_xmit_zc() always did before this
> patch. After this patch, the return value depends on the nb_pkts
> value.
> Two cases might happen then:
> 1. if (nb_pkts < budget), it means we process all the possible data,
> so
> return true and no more necessary poll will be triggered because of
> this.
> 2. if (nb_pkts == budget), it means we might have more data, so return
> false to let another poll run again.
>
> Fixes: f8e284a02afc ("igb: Add AF_XDP zero-copy Tx support")
> Signed-off-by: Jason Xing <kernelxing@xxxxxxxxxxx>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@xxxxxxxxx>
> ---
> v2
> Link: https://lore.kernel.org/all/20250721083343.16482-1-
> kerneljasonxing@xxxxxxxxx/
> 1. target net tree instead of net-next
> 2. use for loop instead
> ---
> drivers/net/ethernet/intel/igb/igb_xsk.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/net/ethernet/intel/igb/igb_xsk.c
> b/drivers/net/ethernet/intel/igb/igb_xsk.c
> index 5cf67ba29269..30ce5fbb5b77 100644
> --- a/drivers/net/ethernet/intel/igb/igb_xsk.c
> +++ b/drivers/net/ethernet/intel/igb/igb_xsk.c
> @@ -482,7 +482,7 @@ bool igb_xmit_zc(struct igb_ring *tx_ring, struct
> xsk_buff_pool *xsk_pool)
> if (!nb_pkts)
> return true;
>
> - while (nb_pkts-- > 0) {
> + for (; i < nb_pkts; i++) {
> dma = xsk_buff_raw_get_dma(xsk_pool, descs[i].addr);
> xsk_buff_raw_dma_sync_for_device(xsk_pool, dma,
> descs[i].len);
>
> @@ -512,7 +512,6 @@ bool igb_xmit_zc(struct igb_ring *tx_ring, struct
> xsk_buff_pool *xsk_pool)
>
> total_bytes += descs[i].len;
>
> - i++;
> tx_ring->next_to_use++;
> tx_buffer_info->next_to_watch = tx_desc;
> if (tx_ring->next_to_use == tx_ring->count)
> --
> 2.41.3
