netdev - Re: [xdp-hints] [RFC bpf-next 05/23] ice: Introduce ice_xdp

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Message-ID: <ZPncfkACKhPFU0PU@lincoln>
Date: Thu, 7 Sep 2023 16:21:50 +0200
From: Larysa Zaremba <larysa.zaremba@...el.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
CC: <bpf@...r.kernel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
	<andrii@...nel.org>, <martin.lau@...ux.dev>, <song@...nel.org>, <yhs@...com>,
	<john.fastabend@...il.com>, <kpsingh@...nel.org>, <sdf@...gle.com>,
	<haoluo@...gle.com>, <jolsa@...nel.org>, David Ahern <dsahern@...il.com>,
	Jakub Kicinski <kuba@...nel.org>, Willem de Bruijn <willemb@...gle.com>,
	Jesper Dangaard Brouer <brouer@...hat.com>, Anatoly Burakov
	<anatoly.burakov@...el.com>, Alexander Lobakin <alexandr.lobakin@...el.com>,
	Magnus Karlsson <magnus.karlsson@...il.com>, Maryam Tahhan
	<mtahhan@...hat.com>, <xdp-hints@...-project.net>, <netdev@...r.kernel.org>,
	Willem de Bruijn <willemdebruijn.kernel@...il.com>, Alexei Starovoitov
	<alexei.starovoitov@...il.com>, Simon Horman <simon.horman@...igine.com>,
	Tariq Toukan <tariqt@...lanox.com>, Saeed Mahameed <saeedm@...lanox.com>
Subject: Re: [xdp-hints] [RFC bpf-next 05/23] ice: Introduce ice_xdp_buff

On Tue, Sep 05, 2023 at 07:53:03PM +0200, Maciej Fijalkowski wrote:
> On Mon, Sep 04, 2023 at 08:11:09PM +0200, Larysa Zaremba wrote:
> > On Mon, Sep 04, 2023 at 05:32:14PM +0200, Maciej Fijalkowski wrote:
> > > On Thu, Aug 24, 2023 at 09:26:44PM +0200, Larysa Zaremba wrote:
> > > > In order to use XDP hints via kfuncs we need to put
> > > > RX descriptor and ring pointers just next to xdp_buff.
> > > > Same as in hints implementations in other drivers, we achieve
> > > > this through putting xdp_buff into a child structure.
> > > 
> > > Don't you mean a parent struct? xdp_buff will be 'child' of ice_xdp_buff
> > > if i'm reading this right.
> > >
> > 
> > ice_xdp_buff is a child in terms of inheritance (pointer to ice_xdp_buff could 
> > replace pointer to xdp_buff, but not in reverse).
> > 
> > > > 
> > > > Currently, xdp_buff is stored in the ring structure,
> > > > so replace it with union that includes child structure.
> > > > This way enough memory is available while existing XDP code
> > > > remains isolated from hints.
> > > > 
> > > > Minimum size of the new child structure (ice_xdp_buff) is exactly
> > > > 64 bytes (single cache line). To place it at the start of a cache line,
> > > > move 'next' field from CL1 to CL3, as it isn't used often. This still
> > > > leaves 128 bits available in CL3 for packet context extensions.
> > > 
> > > I believe ice_xdp_buff will be beefed up in later patches, so what is the
> > > point of moving 'next' ? We won't be able to keep ice_xdp_buff in a single
> > > CL anyway.
> > >
> > 
> > It is to at least keep xdp_buff and descriptor pointer (used for every hint) in 
> > a single CL, other fields are situational.
> 
> Right, something must be moved...still, would be good to see perf
> before/after :)
> 
> > 
> > > > 
> > > > Signed-off-by: Larysa Zaremba <larysa.zaremba@...el.com>
> > > > ---
> > > >  drivers/net/ethernet/intel/ice/ice_txrx.c     |  7 +++--
> > > >  drivers/net/ethernet/intel/ice/ice_txrx.h     | 26 ++++++++++++++++---
> > > >  drivers/net/ethernet/intel/ice/ice_txrx_lib.h | 10 +++++++
> > > >  3 files changed, 38 insertions(+), 5 deletions(-)
> > > > 
> > > > diff --git a/drivers/net/ethernet/intel/ice/ice_txrx.c b/drivers/net/ethernet/intel/ice/ice_txrx.c
> > > > index 40f2f6dabb81..4e6546d9cf85 100644
> > > > --- a/drivers/net/ethernet/intel/ice/ice_txrx.c
> > > > +++ b/drivers/net/ethernet/intel/ice/ice_txrx.c
> > > > @@ -557,13 +557,14 @@ ice_rx_frame_truesize(struct ice_rx_ring *rx_ring, const unsigned int size)
> > > >   * @xdp_prog: XDP program to run
> > > >   * @xdp_ring: ring to be used for XDP_TX action
> > > >   * @rx_buf: Rx buffer to store the XDP action
> > > > + * @eop_desc: Last descriptor in packet to read metadata from
> > > >   *
> > > >   * Returns any of ICE_XDP_{PASS, CONSUMED, TX, REDIR}
> > > >   */
> > > >  static void
> > > >  ice_run_xdp(struct ice_rx_ring *rx_ring, struct xdp_buff *xdp,
> > > >  	    struct bpf_prog *xdp_prog, struct ice_tx_ring *xdp_ring,
> > > > -	    struct ice_rx_buf *rx_buf)
> > > > +	    struct ice_rx_buf *rx_buf, union ice_32b_rx_flex_desc *eop_desc)
> > > >  {
> > > >  	unsigned int ret = ICE_XDP_PASS;
> > > >  	u32 act;
> > > > @@ -571,6 +572,8 @@ ice_run_xdp(struct ice_rx_ring *rx_ring, struct xdp_buff *xdp,
> > > >  	if (!xdp_prog)
> > > >  		goto exit;
> > > >  
> > > > +	ice_xdp_meta_set_desc(xdp, eop_desc);
> > > 
> > > I am currently not sure if for multi-buffer case HW repeats all the
> > > necessary info within each descriptor for every frag? IOW shouldn't you be
> > > using the ice_rx_ring::first_desc?
> > > 
> > > Would be good to test hints for mbuf case for sure.
> > >
> > 
> > In the skb path, we take metadata from the last descriptor only, so this should 
> > be fine. Really worth testing with mbuf though.

I retract my promise to test this with mbuf, as for now hints and mbuf are not 
supposed to go together [0].

Making sure they can co-exist peacefully can be a topic for another series.
For now I just can just say with high confidence that in case of multi-buffer 
frames, we do have all the supported metadata in the EoP descriptor.

[0] https://elixir.bootlin.com/linux/v6.5.2/source/kernel/bpf/offload.c#L234

> 
> Ok, thanks!
>