Re: [xdp-hints] Re: [PATCH bpf-next v3 2/3] net: stmmac: add Launch Time support to XDP ZC

[Stanislav Fomichev]

On Tue, Dec 5, 2023 at 7:34 AM Florian Bezdeka
<florian.bezdeka@xxxxxxxxxxx> 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) support to XDP zero
> > > > > copy via XDP Tx metadata framework.
> > > > >
> > > > > Signed-off-by: Song Yoong Siang<yoong.siang.song@xxxxxxxxx>
> > > > > ---
> > > > >   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 max 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.  It is natural
> > > > that different types of hardware have limitations.  This is a close-to
> > > > hardware-level abstraction/API, and IMHO as long as we document the
> > > > limitations we can expose this API without too many limitations 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 passed that
> > > cannot be programmed.
> > >
> >
> > In current design, the xsk_tx_metadata_request() dint got return 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?
>
> That is clearly a situation that the user should be informed about. For
> RT systems this normally means that something is really wrong regarding
> timing / cycle overflow. Such systems have to react on that situation.

In general, af_xdp is a bit lacking in this 'notify the user that they
somehow messed up' area :-(
For example, pushing a tx descriptor with a wrong addr/len in zc mode
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?