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Date:   Mon, 21 Dec 2020 09:58:18 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>,
        BPF-dev-list <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Saeed Mahameed <saeed@...nel.org>
Subject: Re: [PATCH v4 bpf-next 1/2] net: xdp: introduce xdp_init_buff utility routine

On Mon, Dec 21, 2020 at 9:37 AM Jesper Dangaard Brouer
<brouer@...hat.com> wrote:
>
> On Sat, 19 Dec 2020 18:55:00 +0100
> Lorenzo Bianconi <lorenzo@...nel.org> wrote:
>
> > diff --git a/include/net/xdp.h b/include/net/xdp.h
> > index 11ec93f827c0..323340caef88 100644
> > --- a/include/net/xdp.h
> > +++ b/include/net/xdp.h
> > @@ -76,6 +76,13 @@ struct xdp_buff {
> >       u32 frame_sz; /* frame size to deduce data_hard_end/reserved tailroom*/
> >  };
> >
> > +static __always_inline void
> > +xdp_init_buff(struct xdp_buff *xdp, u32 frame_sz, struct xdp_rxq_info *rxq)
> > +{
> > +     xdp->frame_sz = frame_sz;
> > +     xdp->rxq = rxq;
>
> Later you will add 'xdp->mb = 0' here.

right

>
> > +}
>
> Via the names of your functions, I assume that xdp_init_buff() is
> called before xdp_prepare_buff(), right?
> (And your pending 'xdp->mb = 0' also prefer this.)
>
> Below in bpf_prog_test_run_xdp() and netif_receive_generic_xdp() you
> violate this order... which will give you headaches when implementing
> the multi-buff support.  It is also a bad example for driver developer
> that need to figure out this calling-order from the function names.
>
> Below, will it be possible to have 'init' before 'prepare'?
>

ack, right. Looking at the code we can fix it and have xdp_init_buff()
before xdp_prepare_buff(). I will fix it in v5.

Regards,
Lorenzo

>
> > +
> >  /* Reserve memory area at end-of data area.
> >   *
> >   * This macro reserves tailroom in the XDP buffer by limiting the
> > diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> > index c1c30a9f76f3..a8fa5a9e4137 100644
> > --- a/net/bpf/test_run.c
> > +++ b/net/bpf/test_run.c
> > @@ -640,10 +640,10 @@ int bpf_prog_test_run_xdp(struct bpf_prog *prog, const union bpf_attr *kattr,
> >       xdp.data = data + headroom;
> >       xdp.data_meta = xdp.data;
> >       xdp.data_end = xdp.data + size;
> > -     xdp.frame_sz = headroom + max_data_sz + tailroom;
> >
> >       rxqueue = __netif_get_rx_queue(current->nsproxy->net_ns->loopback_dev, 0);
> > -     xdp.rxq = &rxqueue->xdp_rxq;
> > +     xdp_init_buff(&xdp, headroom + max_data_sz + tailroom,
> > +                   &rxqueue->xdp_rxq);
> >       bpf_prog_change_xdp(NULL, prog);
> >       ret = bpf_test_run(prog, &xdp, repeat, &retval, &duration, true);
> >       if (ret)
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index a46334906c94..b1a765900c01 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -4588,11 +4588,11 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
> >       struct netdev_rx_queue *rxqueue;
> >       void *orig_data, *orig_data_end;
> >       u32 metalen, act = XDP_DROP;
> > +     u32 mac_len, frame_sz;
> >       __be16 orig_eth_type;
> >       struct ethhdr *eth;
> >       bool orig_bcast;
> >       int hlen, off;
> > -     u32 mac_len;
> >
> >       /* Reinjected packets coming from act_mirred or similar should
> >        * not get XDP generic processing.
> > @@ -4631,8 +4631,8 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
> >       xdp->data_hard_start = skb->data - skb_headroom(skb);
> >
> >       /* SKB "head" area always have tailroom for skb_shared_info */
> > -     xdp->frame_sz  = (void *)skb_end_pointer(skb) - xdp->data_hard_start;
> > -     xdp->frame_sz += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> > +     frame_sz = (void *)skb_end_pointer(skb) - xdp->data_hard_start;
> > +     frame_sz += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> >
> >       orig_data_end = xdp->data_end;
> >       orig_data = xdp->data;
> > @@ -4641,7 +4641,7 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
> >       orig_eth_type = eth->h_proto;
> >
> >       rxqueue = netif_get_rxqueue(skb);
> > -     xdp->rxq = &rxqueue->xdp_rxq;
> > +     xdp_init_buff(xdp, frame_sz, &rxqueue->xdp_rxq);
> >
> >       act = bpf_prog_run_xdp(xdp_prog, xdp);
>
>
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer
>

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