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Date:   Tue, 05 Dec 2023 15:01:47 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To:     Stanislav Fomichev <sdf@google.com>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>
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Subject: Re: [xdp-hints] Re: [PATCH bpf-next v3 2/3] net: stmmac: add Launch
 Time support to XDP ZC
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Stanislav Fomichev wrote:
> On 12/05, Willem de Bruijn wrote:
> > Stanislav Fomichev wrote:
> > > On Tue, Dec 5, 2023 at 7:34=E2=80=AFAM Florian Bezdeka
> > > <florian.bezdeka@siemens.com> wrote:
> > > >
> > > > On Tue, 2023-12-05 at 15:25 +0000, Song, Yoong Siang wrote:
> > > > > On Monday, December 4, 2023 10:55 PM, Willem de Bruijn wrote:
> > > > > > Jesper Dangaard Brouer wrote:
> > > > > > >
> > > > > > >
> > > > > > > On 12/3/23 17:51, Song Yoong Siang wrote:
> > > > > > > > This patch enables Launch Time (Time-Based Scheduling) su=
pport to XDP zero
> > > > > > > > copy via XDP Tx metadata framework.
> > > > > > > >
> > > > > > > > Signed-off-by: Song Yoong Siang<yoong.siang.song@intel.co=
m>
> > > > > > > > ---
> > > > > > > >   drivers/net/ethernet/stmicro/stmmac/stmmac.h      |  2 =
++
> > > > > > >
> > > > > > > As requested before, I think we need to see another driver =
implementing
> > > > > > > this.
> > > > > > >
> > > > > > > I propose driver igc and chip i225.
> > > > >
> > > > > Sure. I will include igc patches in next version.
> > > > >
> > > > > > >
> > > > > > > The interesting thing for me is to see how the LaunchTime m=
ax 1 second
> > > > > > > into the future[1] is handled code wise. One suggestion is =
to add a
> > > > > > > section to Documentation/networking/xsk-tx-metadata.rst per=
 driver that
> > > > > > > mentions/documents these different hardware limitations.  I=
t is natural
> > > > > > > that different types of hardware have limitations.  This is=
 a close-to
> > > > > > > hardware-level abstraction/API, and IMHO as long as we docu=
ment the
> > > > > > > limitations we can expose this API without too many limitat=
ions for more
> > > > > > > capable hardware.
> > > > >
> > > > > Sure. I will try to add hardware limitations in documentation.
> > > > >
> > > > > >
> > > > > > I would assume that the kfunc will fail when a value is passe=
d that
> > > > > > cannot be programmed.
> > > > > >
> > > > >
> > > > > In current design, the xsk_tx_metadata_request() dint got retur=
n value.
> > > > > So user won't know if their request is fail.
> > > > > It is complex to inform user which request is failing.
> > > > > Therefore, IMHO, it is good that we let driver handle the error=
 silently.
> > > > >
> > > >
> > > > If the programmed value is invalid, the packet will be "dropped" =
/ will
> > > > never make it to the wire, right?
> > =

> > Programmable behavior is to either drop or cap to some boundary
> > value, such as the farthest programmable time in the future: the
> > horizon. In fq:
> > =

> >                 /* Check if packet timestamp is too far in the future=
. */
> >                 if (fq_packet_beyond_horizon(skb, q, now)) {
> >                         if (q->horizon_drop) {
> >                                         q->stat_horizon_drops++;
> >                                         return qdisc_drop(skb, sch, t=
o_free);
> >                         }
> >                         q->stat_horizon_caps++;
> >                         skb->tstamp =3D now + q->horizon;
> >                 }
> >                 fq_skb_cb(skb)->time_to_send =3D skb->tstamp;
> > =

> > Drop is the more obviously correct mode.
> > =

> > Programming with a clock source that the driver does not support will=

> > then be a persistent failure.
> > =

> > Preferably, this driver capability can be queried beforehand (rather
> > than only through reading error counters afterwards).
> > =

> > Perhaps it should not be a driver task to convert from possibly
> > multiple clock sources to the device native clock. Right now, we do
> > use per-device timecounters for this, implemented in the driver.
> > =

> > As for which clocks are relevant. For PTP, I suppose the device PHC,
> > converted to nsec. For pacing offload, TCP uses CLOCK_MONOTONIC.
> =

> Do we need to expose some generic netdev netlink apis to query/adjust
> nic clock sources (or maybe there is something existing already)?
> Then the userspace can be responsible for syncing/converting the
> timestamps to the internal nic clocks. +1 to trying to avoid doing
> this in the drivers.

Perhaps. I'm just a bit hesitant since that is UAPI and this is all
quite hand-wavy still.

Some of the conversion necessarily has to be in the driver. Only the
driver knows the descriptor format, and limitations of that, such as
the bit-width that can be encoded.

If we cannot move anything out of the drivers (quite likely), then
agreed that a netdev/ethtool netlink query approach is helpful.

To be clear: I don't mean that that should be part of this series.
This is not an XSK specific concern.

> > > > That is clearly a situation that the user should be informed abou=
t. For
> > > > RT systems this normally means that something is really wrong reg=
arding
> > > > timing / cycle overflow. Such systems have to react on that situa=
tion.
> > > =

> > > In general, af_xdp is a bit lacking in this 'notify the user that t=
hey
> > > somehow messed up' area :-(
> > > For example, pushing a tx descriptor with a wrong addr/len in zc mo=
de
> > > will not give any visible signal back (besides driver potentially
> > > spilling something into dmesg as it was in the mlx case).
> > > We can probably start with having some counters for these events?
> > =

> > This is because the AF_XDP completion queue descriptor format is only=

> > a u64 address?
> =

> Yeah. XDP_COPY mode has the descriptor validation which is exported via=

> recvmsg errno, but zerocopy path seems to be too deep in the stack
> to report something back. And there is no place, as you mention,
> in the completion ring to report the status.
> =

> > Could error conditions be reported on tx completion in the metadata,
> > using xsk_tx_metadata_complete?
> =

> That would be one way to do it, yes. But then the error reporting depen=
ds
> on the metadata opt-in. Having a separate ring to export the errors,
> or having a v2 tx-completions layout with extra 'status' field would al=
so
> work.
> =

> But this seems like something that should be handled separately? Becaus=
e
> we'd have to teach all existing zc drivers to report those errors back
> instead of dropping these descriptors..

Agreed on both points :) A v2 tx-completions that supports status
could be useful. But again, this is out of scope of this specific
launch time feature.