Subject: Re: [PATCH bpf-next V2 1/5] igc: enable and fix RX hash usage by netstack

From: John Fastabend <[email-masked]>
Date: Mon, 24 Apr 2023 12:17:03 -0700

Jesper Dangaard Brouer wrote:
> 
> 
> On 23/04/2023 16.46, John Fastabend wrote:
> > Jesper Dangaard Brouer wrote:
> >> 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.
> >>
> >> The NIC hardware was configured to enable RSS hash info in v5.2 via commit
> >> 2121c2712f82 ("igc: Add multiple receive queues control supporting"), but
> >> forgot to set the NETIF_F_RXHASH feature bit.
> >>
> >> The original implementation of igc_rx_hash() didn't extract the associated
> >> pkt_hash_type, but statically set PKT_HASH_TYPE_L3. The largest portions of
> >> this patch are about extracting the RSS Type from the hardware and mapping
> >> this to enum pkt_hash_types. This was based on Foxville i225 software user
> >> manual rev-1.3.1 and tested on Intel Ethernet Controller I225-LM (rev 03).
> >>
> >> For UDP it's worth noting that RSS (type) hashing have been disabled both for
> >> IPv4 and IPv6 (see IGC_MRQC_RSS_FIELD_IPV4_UDP + IGC_MRQC_RSS_FIELD_IPV6_UDP)
> >> because hardware RSS doesn't handle fragmented pkts well when enabled (can
> >> cause out-of-order). This results in PKT_HASH_TYPE_L3 for UDP packets, and
> >> hash value doesn't include UDP port numbers. Not being PKT_HASH_TYPE_L4, have
> >> the effect that netstack will do a software based hash calc calling into
> >> flow_dissect, but only when code calls skb_get_hash(), which doesn't
> >> necessary happen for local delivery.
> >>
> >> For QA verification testing I wrote a small bpftrace prog:
> >>   [0] https://github.com/xdp-project/xdp-project/blob/master/areas/hints/monitor_skb_hash_on_dev.bt
> >>
> >> Fixes: 2121c2712f82 ("igc: Add multiple receive queues control supporting")
> >> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> >> ---
> >>   drivers/net/ethernet/intel/igc/igc.h      |   28 ++++++++++++++++++++++++++
> >>   drivers/net/ethernet/intel/igc/igc_main.c |   31 +++++++++++++++++++++++++----
> >>   2 files changed, 55 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/net/ethernet/intel/igc/igc.h b/drivers/net/ethernet/intel/igc/igc.h
> >> index 34aebf00a512..f7f9e217e7b4 100644
> >> --- a/drivers/net/ethernet/intel/igc/igc.h
> >> +++ b/drivers/net/ethernet/intel/igc/igc.h
> >> @@ -13,6 +13,7 @@
> >>   #include <linux/ptp_clock_kernel.h>
> >>   #include <linux/timecounter.h>
> >>   #include <linux/net_tstamp.h>
> >> +#include <linux/bitfield.h>
> >>   
> >>   #include "igc_hw.h"
> >>   
> >> @@ -311,6 +312,33 @@ 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		GENMASK(3,0) /* 4-bits (3:0) = mask 0x0F */
> >> +
> >> +/* igc_rss_type - Rx descriptor RSS type field */
> >> +static inline u32 igc_rss_type(const union igc_adv_rx_desc *rx_desc)
> >> +{
> >> +	/* RSS Type 4-bits (3:0) number: 0-9 (above 9 is reserved)
> >> +	 * Accessing the same bits via u16 (wb.lower.lo_dword.hs_rss.pkt_info)
> >> +	 * is slightly slower than via u32 (wb.lower.lo_dword.data)
> >> +	 */
> >> +	return le32_get_bits(rx_desc->wb.lower.lo_dword.data, IGC_RSS_TYPE_MASK);
> >> +}
> >> +
> >>   /* 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 1c4676882082..bfa9768d447f 100644
> >> --- a/drivers/net/ethernet/intel/igc/igc_main.c
> >> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
> >> @@ -1690,14 +1690,36 @@ 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 */
> >> +static const enum pkt_hash_types igc_rss_type_table[IGC_RSS_TYPE_MAX_TABLE] = {
> >> +	[IGC_RSS_TYPE_NO_HASH]		= PKT_HASH_TYPE_L2,
> >> +	[IGC_RSS_TYPE_HASH_TCP_IPV4]	= PKT_HASH_TYPE_L4,
> >> +	[IGC_RSS_TYPE_HASH_IPV4]	= PKT_HASH_TYPE_L3,
> >> +	[IGC_RSS_TYPE_HASH_TCP_IPV6]	= PKT_HASH_TYPE_L4,
> >> +	[IGC_RSS_TYPE_HASH_IPV6_EX]	= PKT_HASH_TYPE_L3,
> >> +	[IGC_RSS_TYPE_HASH_IPV6]	= PKT_HASH_TYPE_L3,
> >> +	[IGC_RSS_TYPE_HASH_TCP_IPV6_EX] = PKT_HASH_TYPE_L4,
> >> +	[IGC_RSS_TYPE_HASH_UDP_IPV4]	= PKT_HASH_TYPE_L4,
> >> +	[IGC_RSS_TYPE_HASH_UDP_IPV6]	= PKT_HASH_TYPE_L4,
> >> +	[IGC_RSS_TYPE_HASH_UDP_IPV6_EX] = PKT_HASH_TYPE_L4,
> >> +	[10] = PKT_HASH_TYPE_NONE, /* RSS Type above 9 "Reserved" by HW  */
> >> +	[11] = PKT_HASH_TYPE_NONE, /* keep array sized for SW bit-mask   */
> >> +	[12] = PKT_HASH_TYPE_NONE, /* to handle future HW revisons       */
> >> +	[13] = PKT_HASH_TYPE_NONE,
> >> +	[14] = PKT_HASH_TYPE_NONE,
> >> +	[15] = PKT_HASH_TYPE_NONE,
> >> +};
> >> +
> >>   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) {
> >> +		u32 rss_hash = le32_to_cpu(rx_desc->wb.lower.hi_dword.rss);
> >> +		u32 rss_type = igc_rss_type(rx_desc);
> >> +
> >> +		skb_set_hash(skb, rss_hash, igc_rss_type_table[rss_type]);
> > 
> > Just curious why not copy the logic from the other driver fms10k, ice, ect.
> > 
> > 	skb_set_hash(skb, le32_to_cpu(rx_desc->wb.lower.hi_dword.rss),
> > 		     (IXGBE_RSS_L4_TYPES_MASK & (1ul << rss_type)) ?
> > 		     PKT_HASH_TYPE_L4 : PKT_HASH_TYPE_L3);
> 
> Detail: This code mis-categorize (e.g. ARP) PKT_HASH_TYPE_L2 as
> PKT_HASH_TYPE_L3, but as core reduces this further to one SKB bit, it
> doesn't really matter.
> 
> > avoiding the table logic. Do the driver folks care?
> 
> The define IXGBE_RSS_L4_TYPES_MASK becomes the "table" logic as a 1-bit
> true/false table.  It is a more compact table, let me know if this is
> preferred.
> 
> Yes, it is really upto driver maintainer people to decide, what code is
> preferred ?

Yeah doesn't matter much to me either way. I was just looking at code
compared to ice driver while reviewing.

> --Jesper