[xdp-hints] Re: [PATCH bpf-next V1] igc: enable and fix RX hash usage by netstack

[Alexander Lobakin <alexandr.lobakin@intel.com>]

From: Jesper Dangaard Brouer <brouer@redhat.com>
Date: Fri, 10 Feb 2023 16:07:59 +0100

> When function igc_rx_hash() was introduced in v4.20 via commit 0507ef8a0372
> ("igc: Add transmit and receive fastpath and interrupt handlers"), the
> hardware wasn't configured to provide RSS hash, thus it made sense to not
> enable net_device NETIF_F_RXHASH feature bit.

[...]

> @@ -311,6 +311,58 @@ extern char igc_driver_name[];
>  #define IGC_MRQC_RSS_FIELD_IPV4_UDP	0x00400000
>  #define IGC_MRQC_RSS_FIELD_IPV6_UDP	0x00800000
>  
> +/* RX-desc Write-Back format RSS Type's */
> +enum igc_rss_type_num {
> +	IGC_RSS_TYPE_NO_HASH		= 0,
> +	IGC_RSS_TYPE_HASH_TCP_IPV4	= 1,
> +	IGC_RSS_TYPE_HASH_IPV4		= 2,
> +	IGC_RSS_TYPE_HASH_TCP_IPV6	= 3,
> +	IGC_RSS_TYPE_HASH_IPV6_EX	= 4,
> +	IGC_RSS_TYPE_HASH_IPV6		= 5,
> +	IGC_RSS_TYPE_HASH_TCP_IPV6_EX	= 6,
> +	IGC_RSS_TYPE_HASH_UDP_IPV4	= 7,
> +	IGC_RSS_TYPE_HASH_UDP_IPV6	= 8,
> +	IGC_RSS_TYPE_HASH_UDP_IPV6_EX	= 9,
> +	IGC_RSS_TYPE_MAX		= 10,
> +};
> +#define IGC_RSS_TYPE_MAX_TABLE		16
> +#define IGC_RSS_TYPE_MASK		0xF

GENMASK()?

> +
> +/* igc_rss_type - Rx descriptor RSS type field */
> +static inline u8 igc_rss_type(union igc_adv_rx_desc *rx_desc)

Why use types shorter than u32 on the stack?
Why this union is not const here, since there are no modifications?

> +{
> +	/* RSS Type 4-bit number: 0-9 (above 9 is reserved) */
> +	return rx_desc->wb.lower.lo_dword.hs_rss.pkt_info & IGC_RSS_TYPE_MASK;

The most important I wanted to mention: doesn't this function make the
CPU read the uncached field again, while you could just read it once
onto the stack and then extract all such data from there?

> +}
> +
> +/* Packet header type identified by hardware (when BIT(11) is zero).
> + * Even when UDP ports are not part of RSS hash HW still parse and mark UDP bits
> + */
> +enum igc_pkt_type_bits {
> +	IGC_PKT_TYPE_HDR_IPV4	=	BIT(0),
> +	IGC_PKT_TYPE_HDR_IPV4_WITH_OPT=	BIT(1), /* IPv4 Hdr includes IP options */
> +	IGC_PKT_TYPE_HDR_IPV6	=	BIT(2),
> +	IGC_PKT_TYPE_HDR_IPV6_WITH_EXT=	BIT(3), /* IPv6 Hdr includes extensions */
> +	IGC_PKT_TYPE_HDR_L4_TCP	=	BIT(4),
> +	IGC_PKT_TYPE_HDR_L4_UDP	=	BIT(5),
> +	IGC_PKT_TYPE_HDR_L4_SCTP=	BIT(6),
> +	IGC_PKT_TYPE_HDR_NFS	=	BIT(7),
> +	/* Above only valid when BIT(11) is zero */
> +	IGC_PKT_TYPE_L2		=	BIT(11),
> +	IGC_PKT_TYPE_VLAN	=	BIT(12),
> +	IGC_PKT_TYPE_MASK	=	0x1FFF, /* 13-bits */

Also GENMASK().

> +};
> +
> +/* igc_pkt_type - Rx descriptor Packet type field */
> +static inline u16 igc_pkt_type(union igc_adv_rx_desc *rx_desc)

Also short types and consts.

> +{
> +	u32 data = le32_to_cpu(rx_desc->wb.lower.lo_dword.data);
> +	/* Packet type is 13-bits - as bits (16:4) in lower.lo_dword*/
> +	u16 pkt_type = (data >> 4) & IGC_PKT_TYPE_MASK;

Perfect candidate for FIELD_GET(). No, even for le32_get_bits().

Also my note above about excessive expensive reads.

> +
> +	return pkt_type;
> +}
> +
>  /* Interrupt defines */
>  #define IGC_START_ITR			648 /* ~6000 ints/sec */
>  #define IGC_4K_ITR			980
> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
> index 8b572cd2c350..42a072509d2a 100644
> --- a/drivers/net/ethernet/intel/igc/igc_main.c
> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
> @@ -1677,14 +1677,40 @@ static void igc_rx_checksum(struct igc_ring *ring,
>  		   le32_to_cpu(rx_desc->wb.upper.status_error));
>  }
>  
> +/* Mapping HW RSS Type to enum pkt_hash_types */
> +struct igc_rss_type {
> +	u8 hash_type; /* can contain enum pkt_hash_types */

Why make a struct for one field? + short type note

> +} igc_rss_type_table[IGC_RSS_TYPE_MAX_TABLE] = {
> +	[IGC_RSS_TYPE_NO_HASH].hash_type	  = PKT_HASH_TYPE_L2,
> +	[IGC_RSS_TYPE_HASH_TCP_IPV4].hash_type	  = PKT_HASH_TYPE_L4,
> +	[IGC_RSS_TYPE_HASH_IPV4].hash_type	  = PKT_HASH_TYPE_L3,
> +	[IGC_RSS_TYPE_HASH_TCP_IPV6].hash_type	  = PKT_HASH_TYPE_L4,
> +	[IGC_RSS_TYPE_HASH_IPV6_EX].hash_type	  = PKT_HASH_TYPE_L3,
> +	[IGC_RSS_TYPE_HASH_IPV6].hash_type	  = PKT_HASH_TYPE_L3,
> +	[IGC_RSS_TYPE_HASH_TCP_IPV6_EX].hash_type = PKT_HASH_TYPE_L4,
> +	[IGC_RSS_TYPE_HASH_UDP_IPV4].hash_type	  = PKT_HASH_TYPE_L4,
> +	[IGC_RSS_TYPE_HASH_UDP_IPV6].hash_type	  = PKT_HASH_TYPE_L4,
> +	[IGC_RSS_TYPE_HASH_UDP_IPV6_EX].hash_type = PKT_HASH_TYPE_L4,
> +	[10].hash_type = PKT_HASH_TYPE_L2, /* RSS Type above 9 "Reserved" by HW */
> +	[11].hash_type = PKT_HASH_TYPE_L2,
> +	[12].hash_type = PKT_HASH_TYPE_L2,
> +	[13].hash_type = PKT_HASH_TYPE_L2,
> +	[14].hash_type = PKT_HASH_TYPE_L2,
> +	[15].hash_type = PKT_HASH_TYPE_L2,

Why define those empty if you could do a bound check in the code
instead? E.g. `if (unlikely(bigger_than_9)) return PKT_HASH_TYPE_L2`.

> +};
> +
>  static inline void igc_rx_hash(struct igc_ring *ring,
>  			       union igc_adv_rx_desc *rx_desc,
>  			       struct sk_buff *skb)
>  {
> -	if (ring->netdev->features & NETIF_F_RXHASH)
> -		skb_set_hash(skb,
> -			     le32_to_cpu(rx_desc->wb.lower.hi_dword.rss),
> -			     PKT_HASH_TYPE_L3);
> +	if (ring->netdev->features & NETIF_F_RXHASH) {

	if (!(feature & HASH))
		return;

and -1 indent level?

> +		u32 rss_hash = le32_to_cpu(rx_desc->wb.lower.hi_dword.rss);
> +		u8  rss_type = igc_rss_type(rx_desc);
> +		enum pkt_hash_types hash_type;
> +
> +		hash_type = igc_rss_type_table[rss_type].hash_type;
> +		skb_set_hash(skb, rss_hash, hash_type);
> +	}
>  }

[...]

Thanks,
Olek