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

[Florian Bezdeka <florian.bezdeka@siemens.com>]

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@intel.com>
> > > > ---
> > > >   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.

>  
> 
> > What is being implemented here already exists for qdiscs. The FQ
> > qdisc takes a horizon attribute and
> > 
> >    "
> >    when a packet is beyond the horizon
> >        at enqueue() time:
> >        - either drop the packet (default policy)
> >        - or cap its delivery time to the horizon.
> >    "
> >    commit 39d010504e6b ("net_sched: sch_fq: add horizon attribute")
> > 
> > Having the admin manually configure this on the qdisc based on
> > off-line knowledge of the device is more fragile than if the device
> > would somehow signal its limit to the stack.
> > 
> > But I don't think we should add enforcement of that as a requirement
> > for this xdp extension of pacing.