Subject: Re: [PATCH bpf-next v6 07/18] ice: Support XDP hints in AF_XDP ZC mode

From: Maciej Fijalkowski <[email-masked]>
Date: Tue, 17 Oct 2023 18:45:02 +0200

On Tue, Oct 17, 2023 at 06:37:07PM +0200, Magnus Karlsson wrote:
> On Tue, 17 Oct 2023 at 18:13, Maciej Fijalkowski
> <[email-masked]> wrote:
> >
> > On Thu, Oct 12, 2023 at 07:05:13PM +0200, Larysa Zaremba wrote:
> > > In AF_XDP ZC, xdp_buff is not stored on ring,
> > > instead it is provided by xsk_buff_pool.
> > > Space for metadata sources right after such buffers was already reserved
> > > in commit 94ecc5ca4dbf ("xsk: Add cb area to struct xdp_buff_xsk").
> > > This makes the implementation rather straightforward.
> > >
> > > Update AF_XDP ZC packet processing to support XDP hints.
> > >
> > > Signed-off-by: Larysa Zaremba <[email-masked]>
> > > ---
> > >  drivers/net/ethernet/intel/ice/ice_xsk.c | 34 ++++++++++++++++++++++--
> > >  1 file changed, 32 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/net/ethernet/intel/ice/ice_xsk.c b/drivers/net/ethernet/intel/ice/ice_xsk.c
> > > index ef778b8e6d1b..6ca620b2fbdd 100644
> > > --- a/drivers/net/ethernet/intel/ice/ice_xsk.c
> > > +++ b/drivers/net/ethernet/intel/ice/ice_xsk.c
> > > @@ -752,22 +752,51 @@ static int ice_xmit_xdp_tx_zc(struct xdp_buff *xdp,
> > >       return ICE_XDP_CONSUMED;
> > >  }
> > >
> > > +/**
> > > + * ice_prepare_pkt_ctx_zc - Prepare packet context for XDP hints
> > > + * @xdp: xdp_buff used as input to the XDP program
> > > + * @eop_desc: End of packet descriptor
> > > + * @rx_ring: Rx ring with packet context
> > > + *
> > > + * In regular XDP, xdp_buff is placed inside the ring structure,
> > > + * just before the packet context, so the latter can be accessed
> > > + * with xdp_buff address only at all times, but in ZC mode,
> > > + * xdp_buffs come from the pool, so we need to reinitialize
> > > + * context for every packet.
> > > + *
> > > + * We can safely convert xdp_buff_xsk to ice_xdp_buff,
> > > + * because there are XSK_PRIV_MAX bytes reserved in xdp_buff_xsk
> > > + * right after xdp_buff, for our private use.
> > > + * XSK_CHECK_PRIV_TYPE() ensures we do not go above the limit.
> > > + */
> > > +static void ice_prepare_pkt_ctx_zc(struct xdp_buff *xdp,
> > > +                                union ice_32b_rx_flex_desc *eop_desc,
> > > +                                struct ice_rx_ring *rx_ring)
> > > +{
> > > +     XSK_CHECK_PRIV_TYPE(struct ice_xdp_buff);
> > > +     ((struct ice_xdp_buff *)xdp)->pkt_ctx = rx_ring->pkt_ctx;
> >
> > I will be loud thinking over here, but this could be set in
> > ice_fill_rx_descs(), while grabbing xdp_buffs from xsk_pool, should
> > minimize the performance overhead.
> >
> > But then again you address that with static branch in later patch.
> >
> > OTOH, I was thinking that we could come with xsk_buff_pool API that would
> > let drivers assign this at setup time. Similar what is being done with dma
> > mappings.
> >
> > Magnus, do you think it is worth the hassle? Thoughts?
> 
> I would measure the overhead of the current assignment and if it is
> significant (incurs a cache miss for example), then why not try out
> your idea. Usually good not to have to touch things when not needed.

Larysa measured that because I asked for that previously and impact was
around 6%. Then look at patch 11/18 how this was addressed.

Other ZC drivers didn't report the impact but i am rather sure they were also
affected. So i was thinking whether we should have some generic solution
within pool or every ZC driver handles that on its own.

> 
> > Or should we advise any other driver that support hints to mimic static
> > branch solution?
> >
> > > +     ice_xdp_meta_set_desc(xdp, eop_desc);
> > > +}
> > > +
> > >  /**
> > >   * ice_run_xdp_zc - Executes an XDP program in zero-copy path
> > >   * @rx_ring: Rx ring
> > >   * @xdp: xdp_buff used as input to the XDP program
> > >   * @xdp_prog: XDP program to run
> > >   * @xdp_ring: ring to be used for XDP_TX action
> > > + * @rx_desc: packet descriptor
> > >   *
> > >   * Returns any of ICE_XDP_{PASS, CONSUMED, TX, REDIR}
> > >   */
> > >  static int
> > >  ice_run_xdp_zc(struct ice_rx_ring *rx_ring, struct xdp_buff *xdp,
> > > -            struct bpf_prog *xdp_prog, struct ice_tx_ring *xdp_ring)
> > > +            struct bpf_prog *xdp_prog, struct ice_tx_ring *xdp_ring,
> > > +            union ice_32b_rx_flex_desc *rx_desc)
> > >  {
> > >       int err, result = ICE_XDP_PASS;
> > >       u32 act;
> > >
> > > +     ice_prepare_pkt_ctx_zc(xdp, rx_desc, rx_ring);
> > >       act = bpf_prog_run_xdp(xdp_prog, xdp);
> > >
> > >       if (likely(act == XDP_REDIRECT)) {
> > > @@ -907,7 +936,8 @@ int ice_clean_rx_irq_zc(struct ice_rx_ring *rx_ring, int budget)
> > >               if (ice_is_non_eop(rx_ring, rx_desc))
> > >                       continue;
> > >
> > > -             xdp_res = ice_run_xdp_zc(rx_ring, first, xdp_prog, xdp_ring);
> > > +             xdp_res = ice_run_xdp_zc(rx_ring, first, xdp_prog, xdp_ring,
> > > +                                      rx_desc);
> > >               if (likely(xdp_res & (ICE_XDP_TX | ICE_XDP_REDIR))) {
> > >                       xdp_xmit |= xdp_res;
> > >               } else if (xdp_res == ICE_XDP_EXIT) {
> > > --
> > > 2.41.0
> > >