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

From: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
To: Shay Agroskin <shayagr@amazon.com>
Cc: Lorenzo Bianconi <lorenzo@kernel.org>,
	BPF-dev-list <bpf@vger.kernel.org>,
	Network Development <netdev@vger.kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	"Jubran, Samih" <sameehj@amazon.com>,
	John Fastabend <john.fastabend@gmail.com>,
	David Ahern <dsahern@kernel.org>,
	Jesper Brouer <brouer@redhat.com>,
	Eelco Chaudron <echaudro@redhat.com>,
	Jason Wang <jasowang@redhat.com>
Subject: Re: [PATCH v5 bpf-next 06/14] net: mvneta: add multi buffer support to XDP_TX
Date: Sun, 20 Dec 2020 19:06:56 +0100	[thread overview]
Message-ID: <CAJ0CqmXB1yUzBAxjeyxDw2smpOMXqN=3TdqwwAkr49k8-6x8qA@mail.gmail.com> (raw)
In-Reply-To: <pj41zlh7ohpz6h.fsf@u68c7b5b1d2d758.ant.amazon.com>

On Sat, Dec 19, 2020 at 4:56 PM Shay Agroskin <shayagr@amazon.com> wrote:
>
>
> Lorenzo Bianconi <lorenzo@kernel.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@kernel.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
>
> [...]
>