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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Stanislav Fomichev <stfomichev@gmail.com> writes:

> 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 "HW timestamp has been copied into local variable" comment is outdated on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 :-/

Yeah, I don't think we can impose UAPI restrictions on the metadata area
at this point. I guess the best we can do is to educate users that they
should call the timestamp kfunc before they modify the metadata?

-Toke