netdev - Re: [PATCH net v3] igb: Fix XDP with PTP enabled

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Date:   Fri, 23 Apr 2021 13:51:50 +0200
From:   Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To:     Kurt Kanzenbach <kurt@...utronix.de>
Cc:     Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Sven Auhagen <sven.auhagen@...eatech.de>,
        intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Richard Cochran <richardcochran@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH net v3] igb: Fix XDP with PTP enabled

On Fri, Apr 23, 2021 at 08:45:52AM +0200, Kurt Kanzenbach wrote:
> On Thu Apr 22 2021, Maciej Fijalkowski wrote:
> > On Thu, Apr 22, 2021 at 07:26:17AM +0200, Kurt Kanzenbach wrote:
> >> +		/* pull rx packet timestamp if available and valid */
> >> +		if (igb_test_staterr(rx_desc, E1000_RXDADV_STAT_TSIP)) {
> >> +			timestamp = igb_ptp_rx_pktstamp(rx_ring->q_vector,
> >> +							pktbuf);
> >> +
> >> +			if (timestamp) {
> >> +				pkt_offset += IGB_TS_HDR_LEN;
> >> +				size -= IGB_TS_HDR_LEN;
> >> +			}
> >> +		}
> >
> > Small nit: since this is a hot path, maybe we could omit the additional
> > branch that you're introducing above and make igb_ptp_rx_pktstamp() to
> > return either 0 for error cases and IGB_TS_HDR_LEN if timestamp was fine?
> > timestamp itself would be passed as an arg.
> >
> > So:
> > 		if (igb_test_staterr(rx_desc, E1000_RXDADV_STAT_TSIP)) {
> > 			ts_offset = igb_ptp_rx_pktstamp(rx_ring->q_vector,
> > 							pktbuf, &timestamp);
> > 			pkt_offset += ts_offset;
> > 			size -= ts_offset;
> > 		}
> >
> > Thoughts? I feel like if we see that desc has timestamp enabled then let's
> > optimize it for successful case.
> 
> Yes, this should work as well. Actually I didn't like the if statement
> either. Only one comment: It's not an offset but rather the timestamp
> header length. I'd call it 'ts_len'.

Right, sorry.

> 
> >
> >>  
> >>  		/* retrieve a buffer from the ring */
> >>  		if (!skb) {
> >> -			unsigned int offset = igb_rx_offset(rx_ring);
> >> -			unsigned char *hard_start;
> >> +			unsigned char *hard_start = pktbuf - igb_rx_offset(rx_ring);
> >> +			unsigned int offset = pkt_offset + igb_rx_offset(rx_ring);
> >
> > Probably we could do something similar in flavour of:
> > https://lore.kernel.org/bpf/20210118151318.12324-10-maciej.fijalkowski@intel.com/
> >
> > which broke XDP_REDIRECT and got fixed in:
> > https://lore.kernel.org/bpf/20210303153928.11764-2-maciej.fijalkowski@intel.com/
> >
> > You get the idea.
> 
> Yes, I do. However, I think such a change doesn't belong in this patch,
> which is a bugfix for XDP. It looks like an optimization. Should I split
> it into two patches and rather target net-next instead of net?

This was just a heads up from my side as it caught my eye. For sure it's
out of the scope of that patch, but would be good to have a follow up on
that.

> 
> Thanks for your review.
> 
> Thanks,
> Kurt