Subject: Re: [xdp-hints] [RFC bpf-next 05/23] ice: Introduce ice_xdp_buff

From: Larysa Zaremba <[email-masked]>
Date: Mon, 4 Sep 2023 20:11:09 +0200

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.

> > 
> > Signed-off-by: Larysa Zaremba <[email-masked]>
> > ---
> >  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.

> > +
> >  	act = bpf_prog_run_xdp(xdp_prog, xdp);
> >  	switch (act) {
> >  	case XDP_PASS:
> > @@ -1240,7 +1243,7 @@ int ice_clean_rx_irq(struct ice_rx_ring *rx_ring, int budget)
> >  		if (ice_is_non_eop(rx_ring, rx_desc))
> >  			continue;
> >  
> > -		ice_run_xdp(rx_ring, xdp, xdp_prog, xdp_ring, rx_buf);
> > +		ice_run_xdp(rx_ring, xdp, xdp_prog, xdp_ring, rx_buf, rx_desc);
> >  		if (rx_buf->act == ICE_XDP_PASS)
> >  			goto construct_skb;
> >  		total_rx_bytes += xdp_get_buff_len(xdp);
> > diff --git a/drivers/net/ethernet/intel/ice/ice_txrx.h b/drivers/net/ethernet/intel/ice/ice_txrx.h
> > index 166413fc33f4..d0ab2c4c0c91 100644
> > --- a/drivers/net/ethernet/intel/ice/ice_txrx.h
> > +++ b/drivers/net/ethernet/intel/ice/ice_txrx.h
> > @@ -257,6 +257,18 @@ enum ice_rx_dtype {
> >  	ICE_RX_DTYPE_SPLIT_ALWAYS	= 2,
> >  };
> >  
> > +struct ice_pkt_ctx {
> > +	const union ice_32b_rx_flex_desc *eop_desc;
> > +};
> > +
> > +struct ice_xdp_buff {
> > +	struct xdp_buff xdp_buff;
> > +	struct ice_pkt_ctx pkt_ctx;
> > +};
> > +
> > +/* Required for compatibility with xdp_buffs from xsk_pool */
> > +static_assert(offsetof(struct ice_xdp_buff, xdp_buff) == 0);
> > +
> >  /* indices into GLINT_ITR registers */
> >  #define ICE_RX_ITR	ICE_IDX_ITR0
> >  #define ICE_TX_ITR	ICE_IDX_ITR1
> > @@ -298,7 +310,6 @@ enum ice_dynamic_itr {
> >  /* descriptor ring, associated with a VSI */
> >  struct ice_rx_ring {
> >  	/* CL1 - 1st cacheline starts here */
> > -	struct ice_rx_ring *next;	/* pointer to next ring in q_vector */
> >  	void *desc;			/* Descriptor ring memory */
> >  	struct device *dev;		/* Used for DMA mapping */
> >  	struct net_device *netdev;	/* netdev ring maps to */
> > @@ -310,12 +321,19 @@ struct ice_rx_ring {
> >  	u16 count;			/* Number of descriptors */
> >  	u16 reg_idx;			/* HW register index of the ring */
> >  	u16 next_to_alloc;
> > -	/* CL2 - 2nd cacheline starts here */
> > +
> >  	union {
> >  		struct ice_rx_buf *rx_buf;
> >  		struct xdp_buff **xdp_buf;
> >  	};
> > -	struct xdp_buff xdp;
> > +	/* CL2 - 2nd cacheline starts here */
> > +	union {
> > +		struct ice_xdp_buff xdp_ext;
> > +		struct {
> > +			struct xdp_buff xdp;
> > +			struct ice_pkt_ctx pkt_ctx;
> > +		};
> > +	};
> >  	/* CL3 - 3rd cacheline starts here */
> >  	struct bpf_prog *xdp_prog;
> >  	u16 rx_offset;
> > @@ -325,6 +343,8 @@ struct ice_rx_ring {
> >  	u16 next_to_clean;
> >  	u16 first_desc;
> >  
> > +	struct ice_rx_ring *next;	/* pointer to next ring in q_vector */
> > +
> >  	/* stats structs */
> >  	struct ice_ring_stats *ring_stats;
> >  
> > diff --git a/drivers/net/ethernet/intel/ice/ice_txrx_lib.h b/drivers/net/ethernet/intel/ice/ice_txrx_lib.h
> > index e1d49e1235b3..145883eec129 100644
> > --- a/drivers/net/ethernet/intel/ice/ice_txrx_lib.h
> > +++ b/drivers/net/ethernet/intel/ice/ice_txrx_lib.h
> > @@ -151,4 +151,14 @@ ice_process_skb_fields(struct ice_rx_ring *rx_ring,
> >  		       struct sk_buff *skb);
> >  void
> >  ice_receive_skb(struct ice_rx_ring *rx_ring, struct sk_buff *skb, u16 vlan_tag);
> > +
> > +static inline void
> > +ice_xdp_meta_set_desc(struct xdp_buff *xdp,
> > +		      union ice_32b_rx_flex_desc *eop_desc)
> > +{
> > +	struct ice_xdp_buff *xdp_ext = container_of(xdp, struct ice_xdp_buff,
> > +						    xdp_buff);
> > +
> > +	xdp_ext->pkt_ctx.eop_desc = eop_desc;
> > +}
> >  #endif /* !_ICE_TXRX_LIB_H_ */
> > -- 
> > 2.41.0
> >