Subject: Re: [PATCH net-next 4/4] igb: add AF_XDP zero-copy Tx support

From: Maciej Fijalkowski <[email-masked]>
Date: Wed, 14 Aug 2024 10:55:20 +0200

On Wed, Aug 14, 2024 at 10:36:32AM +0200, Kurt Kanzenbach wrote:
> On Sat Aug 10 2024, Maciej Fijalkowski wrote:
> >> +	nb_pkts = xsk_tx_peek_release_desc_batch(pool, budget);
> >> +	if (!nb_pkts)
> >> +		return true;
> >> +
> >> +	while (nb_pkts-- > 0) {
> >> +		dma = xsk_buff_raw_get_dma(pool, descs[i].addr);
> >> +		xsk_buff_raw_dma_sync_for_device(pool, dma, descs[i].len);
> >> +
> >> +		tx_buffer_info = &tx_ring->tx_buffer_info[tx_ring->next_to_use];
> >> +		tx_buffer_info->bytecount = descs[i].len;
> >> +		tx_buffer_info->type = IGB_TYPE_XSK;
> >> +		tx_buffer_info->xdpf = NULL;
> >> +		tx_buffer_info->gso_segs = 1;
> >> +		tx_buffer_info->time_stamp = jiffies;
> >> +
> >> +		tx_desc = IGB_TX_DESC(tx_ring, tx_ring->next_to_use);
> >> +		tx_desc->read.buffer_addr = cpu_to_le64(dma);
> >> +
> >> +		/* put descriptor type bits */
> >> +		cmd_type = E1000_ADVTXD_DTYP_DATA | E1000_ADVTXD_DCMD_DEXT |
> >> +			   E1000_ADVTXD_DCMD_IFCS;
> >> +		olinfo_status = descs[i].len << E1000_ADVTXD_PAYLEN_SHIFT;
> >> +
> >> +		cmd_type |= descs[i].len | IGB_TXD_DCMD;
> >
> > This is also sub-optimal as you are setting RS bit on each Tx descriptor,
> > which will in turn raise a lot of irqs. See how ice sets RS bit only on
> > last desc from a batch and then, on cleaning side, how it finds a
> > descriptor that is supposed to have DD bit written by HW.
> 
> I see your point. That requires changes to the cleaning side. However,
> igb_clean_tx_irq() is shared between normal and zero-copy path.

Ok if that's too much of a hassle then let's leave it as-is. I can address
that in some nearby future.

> 
> The amount of irqs can be also controlled by irq coalescing or even
> using busy polling. So I'd rather keep this implementation as simple as
> it is now.

That has nothing to do with what I was describing.

> Thanks,
> Kurt