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

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Date:   Fri, 23 Apr 2021 08:45:52 +0200
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
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 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'.

>
>>  
>>  		/* 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?

Thanks for your review.

Thanks,
Kurt

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