Re: [PATCH net-next 2/2] igc: enable XDP metadata in driver

From: Alexander Lobakin <redacted>
Date: Mon, 29 Nov 2021 15:53:03 +0100

From: Jesper Dangaard Brouer <redacted>
Date: Mon, 29 Nov 2021 15:39:04 +0100

quoted

On 26/11/2021 17.16, Alexander Lobakin wrote:

quoted

From: Jesper Dangaard Brouer <redacted>
Date: Mon, 15 Nov 2021 21:36:30 +0100

quoted

Enabling the XDP bpf_prog access to data_meta area is a very small
change. Hint passing 'true' to xdp_prepare_buff().

[ snip ]

quoted

Prefetch works for "full" cachelines. Intel CPUs often prefect two 
cache-lines, when doing this, thus I guess we still get xdp->data.

Sure. I mean, net_prefetch() prefetches 128 bytes in a row.
xdp->data is usually aligned to XDP_PACKET_HEADROOM (or two bytes
to the right). If our CL is 64 and the meta is present, then... ah
right, 64 to the left and 64 starting from data to the right.

quoted

I don't mind prefetching xdp->data_meta, but (1) I tried to keep the 
change minimal as current behavior was data area I kept that. (2) 
xdp->data starts on a cacheline and we know NIC hardware have touched 
that, it is not a full-cache-miss due to DDIO/DCA it is known to be in 
L3 cache (gain is around 2-3 ns in my machine for data prefetch).
Given this is only a 2.5 Gbit/s driver/HW I doubt this make any difference.

Code constistency at least. On 10+ Gbps we prefetch meta, and I plan
to continue doing this in my series.

quoted

Tony is it worth resending a V2 of this patch?

Tony, you can take it as it is if you want, I'll correct it later in
mine. Up to you.

My "fixup" looks like (in case of v2 needed or so):

diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
index b516f1b301b4..142c57b7a451 100644
--- a/drivers/net/ethernet/intel/igc/igc_main.c
+++ b/drivers/net/ethernet/intel/igc/igc_main.c

@@ -1726,7 +1726,7 @@ static struct sk_buff *igc_build_skb(struct igc_ring *rx_ring,
 	struct sk_buff *skb;
 
 	/* prefetch first cache line of first page */
-	net_prefetch(xdp->data);
+	net_prefetch(xdp->data_meta);
 
 	/* build an skb around the page buffer */
 	skb = build_skb(xdp->data_hard_start, truesize);

@@ -1756,10 +1756,11 @@ static struct sk_buff *igc_construct_skb(struct igc_ring *rx_ring,
 	struct sk_buff *skb;
 
 	/* prefetch first cache line of first page */
-	net_prefetch(va);
+	net_prefetch(xdp->data_meta);
 
 	/* allocate a skb to store the frags */
-	skb = napi_alloc_skb(&rx_ring->q_vector->napi, IGC_RX_HDR_LEN + metasize);
+	skb = napi_alloc_skb(&rx_ring->q_vector->napi,
+			     IGC_RX_HDR_LEN + metasize);
 	if (unlikely(!skb))
 		return NULL;
 

@@ -2363,7 +2364,8 @@ static int igc_clean_rx_irq(struct igc_q_vector *q_vector, const int budget)
 		if (!skb) {
 			xdp_init_buff(&xdp, truesize, &rx_ring->xdp_rxq);
 			xdp_prepare_buff(&xdp, pktbuf - igc_rx_offset(rx_ring),
-					 igc_rx_offset(rx_ring) + pkt_offset, size, true);
+					 igc_rx_offset(rx_ring) + pkt_offset,
+					 size, true);
 
 			skb = igc_xdp_run_prog(adapter, &xdp);
 		}

Reviewed-by: Alexander Lobakin <redacted>

quoted

quoted

quoted

  
  	/* build an skb around the page buffer */
-	skb = build_skb(va - IGC_SKB_PAD, truesize);
+	skb = build_skb(xdp->data_hard_start, truesize);
  	if (unlikely(!skb))
  		return NULL;
  
  	/* update pointers within the skb to store the data */
-	skb_reserve(skb, IGC_SKB_PAD);
+	skb_reserve(skb, xdp->data - xdp->data_hard_start);
  	__skb_put(skb, size);
+	if (metasize)
+		skb_metadata_set(skb, metasize);
  
  	igc_rx_buffer_flip(rx_buffer, truesize);
  	return skb;

@@ -1746,6 +1748,7 @@ static struct sk_buff *igc_construct_skb(struct igc_ring *rx_ring,
  					 struct xdp_buff *xdp,
  					 ktime_t timestamp)
  {
+	unsigned int metasize = xdp->data - xdp->data_meta;
  	unsigned int size = xdp->data_end - xdp->data;
  	unsigned int truesize = igc_get_rx_frame_truesize(rx_ring, size);
  	void *va = xdp->data;

@@ -1756,7 +1759,7 @@ static struct sk_buff *igc_construct_skb(struct igc_ring *rx_ring,
  	net_prefetch(va);

...here as well.

Thanks,
Al

Al