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

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Date:   Tue, 05 Dec 2023 16:34:29 +0100
From:   Florian Bezdeka <florian.bezdeka@...mens.com>
To:     "Song, Yoong Siang" <yoong.siang.song@...el.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Bjorn Topel <bjorn@...nel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Mykola Lysenko <mykolal@...com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>, Shuah Khan <shuah@...nel.org>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "xdp-hints@...-project.net" <xdp-hints@...-project.net>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: [xdp-hints] Re: [PATCH bpf-next v3 2/3] net: stmmac: add Launch
 Time support to XDP ZC

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@...el.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.