[xdp-hints] Re: [PATCH bpf-next v6 4/4] igc: Add launch time support to XDP ZC

[Stanislav Fomichev <stfomichev@gmail.com>]

On 01/23, Florian Bezdeka wrote:
> Hi all,
> 
> On Thu, 2025-01-23 at 16:41 +0000, Song, Yoong Siang wrote:
> > On Thursday, January 23, 2025 11:40 PM, Bouska, Zdenek <zdenek.bouska@siemens.com> wrote:
> > > 
> > > Hi Siang,
> > > 
> > > I tested this patch series on 6.13 with Intel I226-LM (rev 04).
> > > 
> > > I also applied patch "selftests/bpf: Actuate tx_metadata_len in xdp_hw_metadata" [1]
> > > and "selftests/bpf: Enable Tx hwtstamp in xdp_hw_metadata" [2] so that TX timestamps
> > > work.
> > > 
> > > HW RX-timestamp was small (0.5956 instead of 1737373125.5956):
> > > 
> > > HW RX-time:   595572448 (sec:0.5956) delta to User RX-time sec:1737373124.9873 (1737373124987318.750 usec)
> > > XDP RX-time:   1737373125582798388 (sec:1737373125.5828) delta to User RX-time sec:0.0001 (92.733 usec)
> > > 
> > > Igc's raw HW RX-timestamp in front of frame data was overwritten by BPF program on
> > > line 90 in tools/testing/selftests/bpf: meta->hint_valid = 0;
> > > 
> > > "HW timestamp has been copied into local variable" comment is outdated on
> > > line 2813 in drivers/net/ethernet/intel/igc/igc_main.c after
> > > commit 069b142f5819 igc: Add support for PTP .getcyclesx64() [3].
> > > 
> > > Workaround is to add unused data to xdp_meta struct:
> > > 
> > > --- a/tools/testing/selftests/bpf/xdp_metadata.h
> > > +++ b/tools/testing/selftests/bpf/xdp_metadata.h
> > > @@ -49,4 +49,5 @@ struct xdp_meta {
> > >                __s32 rx_vlan_tag_err;
> > >        };
> > >        enum xdp_meta_field hint_valid;
> > > +       __u8 avoid_IGC_TS_HDR_LEN[16];
> > > };
> > > 
> > 
> > Hi Zdenek Bouska, 
> > 
> > Thanks for your help on testing this patch set.
> > You are right, there is some issue with the Rx hw timestamp,
> > I will submit the bug fix patch when the solution is finalized,
> > but the fix will not be part of this launch time patch set.
> > Until then, you can continue to use your WA.
> 
> I think there is no simple fix for that. That needs some discussion
> around the "expectations" to the headroom / meta data area in front of
> the actual packet data.

By 'simple' you mean without some new UAPI to signal the size of that
'reserved area' by the driver? I don't see any other easy way out as well :-/