netdev - Re: [PATCH v5 bpf-next 06/14] net: mvneta: add multi buffer support to XDP

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Date:   Sun, 20 Dec 2020 19:06:56 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Shay Agroskin <shayagr@...zon.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>,
        "Jubran, Samih" <sameehj@...zon.com>,
        John Fastabend <john.fastabend@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Jesper Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH v5 bpf-next 06/14] net: mvneta: add multi buffer support
 to XDP_TX

On Sat, Dec 19, 2020 at 4:56 PM Shay Agroskin <shayagr@...zon.com> wrote:
>
>
> Lorenzo Bianconi <lorenzo@...nel.org> writes:
>
> > Introduce the capability to map non-linear xdp buffer running
> > mvneta_xdp_submit_frame() for XDP_TX and XDP_REDIRECT
> >
> > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> > ---
> >  drivers/net/ethernet/marvell/mvneta.c | 94
> >  ++++++++++++++++-----------
> >  1 file changed, 56 insertions(+), 38 deletions(-)
> [...]
> >                       if (napi && buf->type ==
> >  MVNETA_TYPE_XDP_TX)
> >                               xdp_return_frame_rx_napi(buf->xdpf);
> >                       else
> > @@ -2054,45 +2054,64 @@ mvneta_xdp_put_buff(struct mvneta_port
> > *pp, struct mvneta_rx_queue *rxq,
> >
> >  static int
> >  mvneta_xdp_submit_frame(struct mvneta_port *pp, struct
> >  mvneta_tx_queue *txq,
> > -                     struct xdp_frame *xdpf, bool dma_map)
> > +                     struct xdp_frame *xdpf, int *nxmit_byte,
> > bool dma_map)
> >  {
> > -     struct mvneta_tx_desc *tx_desc;
> > -     struct mvneta_tx_buf *buf;
> > -     dma_addr_t dma_addr;
> > +     struct xdp_shared_info *xdp_sinfo =
> > xdp_get_shared_info_from_frame(xdpf);
> > +     int i, num_frames = xdpf->mb ? xdp_sinfo->nr_frags + 1 :
> > 1;
> > +     struct mvneta_tx_desc *tx_desc = NULL;
> > +     struct page *page;
> >
> > -     if (txq->count >= txq->tx_stop_threshold)
> > +     if (txq->count + num_frames >= txq->size)
> >               return MVNETA_XDP_DROPPED;
> >
> > -     tx_desc = mvneta_txq_next_desc_get(txq);
> > +     for (i = 0; i < num_frames; i++) {
> > +             struct mvneta_tx_buf *buf =
> > &txq->buf[txq->txq_put_index];
> > +             skb_frag_t *frag = i ? &xdp_sinfo->frags[i - 1] :
> > NULL;
> > +             int len = frag ? xdp_get_frag_size(frag) :
> > xdpf->len;
>
> nit, from branch prediction point of view, maybe it would be
> better to write
>      int len = i ? xdp_get_frag_size(frag) : xdpf->len;
>

ack, I will fix it in v6.

Regards,
Lorenzo

> since the value of i is checked one line above
> Disclaimer: I'm far from a compiler expert, and don't know whether
> the compiler would know to group these two assignments together
> into a single branch prediction decision, but it feels like using
> 'i' would make this decision easier for it.
>
> Thanks,
> Shay
>
> [...]
>