Re: [PATCH net-next 2/2] igb: xsk: solve underflow of nb_pkts in zerocopy mode

[Simon Horman <horms@kernel.org>]

On Mon, Jul 21, 2025 at 04:33:43PM +0800, Jason Xing wrote:
> From: Jason Xing <kernelxing@tencent.com>
> 
> 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@tencent.com>
> ---
>  drivers/net/ethernet/intel/igb/igb_xsk.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/igb/igb_xsk.c b/drivers/net/ethernet/intel/igb/igb_xsk.c
> index 5cf67ba29269..243f4246fee8 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) {
> +	while (i < nb_pkts) {

Hi Jason,

FWIIW, I think using a for loop is a more idiomatic way
of handling the relationship between i, nb_pkts, and
the iterations of this loop.

>  		dma = xsk_buff_raw_get_dma(xsk_pool, descs[i].addr);
>  		xsk_buff_raw_dma_sync_for_device(xsk_pool, dma, descs[i].len);
>  
> -- 
> 2.41.3
> 
>