Date: Mon, 16 Oct 2023 13:17:49 -0700
From: Alexander H Duyck <alexander.duyck@...il.com>
To: Ahmed Zaki <ahmed.zaki@...el.com>, netdev@...r.kernel.org
Cc: intel-wired-lan@...ts.osuosl.org, corbet@....net, 
 jesse.brandeburg@...el.com, anthony.l.nguyen@...el.com,
 davem@...emloft.net,  edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, vladimir.oltean@....com,  andrew@...n.ch,
 horms@...nel.org, mkubecek@...e.cz,  willemdebruijn.kernel@...il.com,
 linux-doc@...r.kernel.org, Wojciech Drewek <wojciech.drewek@...el.com>
Subject: Re: [PATCH net-next v4 1/6] net: ethtool: allow symmetric-xor RSS
 hash for any flow type

On Mon, 2023-10-16 at 09:49 -0600, Ahmed Zaki wrote:
> Symmetric RSS hash functions are beneficial in applications that monitor
> both Tx and Rx packets of the same flow (IDS, software firewalls, ..etc).
> Getting all traffic of the same flow on the same RX queue results in
> higher CPU cache efficiency.
> 
> A NIC that supports "symmetric-xor" can achieve this RSS hash symmetry
> by XORing the source and destination fields and pass the values to the
> RSS hash algorithm.
> 
> Only fields that has counterparts in the other direction can be
> accepted; IP src/dst and L4 src/dst ports.
> 
> The user may request RSS hash symmetry for a specific flow type, via:
> 
>     # ethtool -N|-U eth0 rx-flow-hash <flow_type> s|d|f|n symmetric-xor
> 
> or turn symmetry off (asymmetric) by:
> 
>     # ethtool -N|-U eth0 rx-flow-hash <flow_type> s|d|f|n
> 
> Reviewed-by: Wojciech Drewek <wojciech.drewek@...el.com>
> Signed-off-by: Ahmed Zaki <ahmed.zaki@...el.com>
> ---
>  Documentation/networking/scaling.rst |  6 ++++++
>  include/uapi/linux/ethtool.h         | 21 +++++++++++++--------
>  net/ethtool/ioctl.c                  | 11 +++++++++++
>  3 files changed, 30 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/networking/scaling.rst b/Documentation/networking/scaling.rst
> index 92c9fb46d6a2..64f3d7566407 100644
> --- a/Documentation/networking/scaling.rst
> +++ b/Documentation/networking/scaling.rst
> @@ -44,6 +44,12 @@ by masking out the low order seven bits of the computed hash for the
>  packet (usually a Toeplitz hash), taking this number as a key into the
>  indirection table and reading the corresponding value.
>  
> +Some NICs support symmetric RSS hashing where, if the IP (source address,
> +destination address) and TCP/UDP (source port, destination port) tuples
> +are swapped, the computed hash is the same. This is beneficial in some
> +applications that monitor TCP/IP flows (IDS, firewalls, ...etc) and need
> +both directions of the flow to land on the same Rx queue (and CPU).
> +
>  Some advanced NICs allow steering packets to queues based on
>  programmable filters. For example, webserver bound TCP port 80 packets
>  can be directed to their own receive queue. Such “n-tuple” filters can
> diff --git a/include/uapi/linux/ethtool.h b/include/uapi/linux/ethtool.h
> index f7fba0dc87e5..4e8d38fb55ce 100644
> --- a/include/uapi/linux/ethtool.h
> +++ b/include/uapi/linux/ethtool.h
> @@ -2018,14 +2018,19 @@ static inline int ethtool_validate_duplex(__u8 duplex)
>  #define	FLOW_RSS	0x20000000
>  
>  /* L3-L4 network traffic flow hash options */
> -#define	RXH_L2DA	(1 << 1)
> -#define	RXH_VLAN	(1 << 2)
> -#define	RXH_L3_PROTO	(1 << 3)
> -#define	RXH_IP_SRC	(1 << 4)
> -#define	RXH_IP_DST	(1 << 5)
> -#define	RXH_L4_B_0_1	(1 << 6) /* src port in case of TCP/UDP/SCTP */
> -#define	RXH_L4_B_2_3	(1 << 7) /* dst port in case of TCP/UDP/SCTP */
> -#define	RXH_DISCARD	(1 << 31)
> +#define	RXH_L2DA		(1 << 1)
> +#define	RXH_VLAN		(1 << 2)
> +#define	RXH_L3_PROTO		(1 << 3)
> +#define	RXH_IP_SRC		(1 << 4)
> +#define	RXH_IP_DST		(1 << 5)
> +#define	RXH_L4_B_0_1		(1 << 6) /* src port in case of TCP/UDP/SCTP */
> +#define	RXH_L4_B_2_3		(1 << 7) /* dst port in case of TCP/UDP/SCTP */
> +/* XOR the corresponding source and destination fields of each specified
> + * protocol. Both copies of the XOR'ed fields are fed into the RSS and RXHASH
> + * calculation.
> + */
> +#define	RXH_SYMMETRIC_XOR	(1 << 30)
> +#define	RXH_DISCARD		(1 << 31)

I guess this has already been discussed but I am not a fan of long
names for defines. I would prefer to see this just be something like
RXH_SYMMETRIC or something like that. The XOR is just an implementation
detail. I have seen the same thing accomplished by just reordering the
fields by min/max approaches.

>  
>  #define	RX_CLS_FLOW_DISC	0xffffffffffffffffULL
>  #define RX_CLS_FLOW_WAKE	0xfffffffffffffffeULL
> diff --git a/net/ethtool/ioctl.c b/net/ethtool/ioctl.c
> index 0b0ce4f81c01..b1bd0d4b48e8 100644
> --- a/net/ethtool/ioctl.c
> +++ b/net/ethtool/ioctl.c
> @@ -980,6 +980,17 @@ static noinline_for_stack int ethtool_set_rxnfc(struct net_device *dev,
>  	if (rc)
>  		return rc;
>  
> +	/* If a symmetric hash is requested, then:
> +	 * 1 - no other fields besides IP src/dst and/or L4 src/dst
> +	 * 2 - If src is set, dst must also be set
> +	 */
> +	if ((info.data & RXH_SYMMETRIC_XOR) &&
> +	    ((info.data & ~(RXH_SYMMETRIC_XOR | RXH_IP_SRC | RXH_IP_DST |
> +	      RXH_L4_B_0_1 | RXH_L4_B_2_3)) ||
> +	     (!!(info.data & RXH_IP_SRC) ^ !!(info.data & RXH_IP_DST)) ||
> +	     (!!(info.data & RXH_L4_B_0_1) ^ !!(info.data & RXH_L4_B_2_3))))
> +		return -EINVAL;
> +
>  	rc = dev->ethtool_ops->set_rxnfc(dev, &info);
>  	if (rc)
>  		return rc;

You are pushing implementation from your device into the interface
design here. You should probably push these requirements down into the
driver rather than making it a part of the generic implementation.

It would be nice to see input from other NIC vendors on this as I
suspect they probably have similar functionality available to them.

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