Ahmed Zaki wrote:

On 2023-08-24 12:36, Saeed Mahameed wrote:

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On 24 Aug 07:14, Ahmed Zaki wrote:

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On 2023-08-23 13:45, Saeed Mahameed wrote:

quoted

On 23 Aug 10:48, Ahmed Zaki wrote:

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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.

...

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What is the expectation of the symmetric toeplitz hash, how do you 
achieve
that? by sorting packet fields? which fields?

Can you please provide a link to documentation/spec?
We should make sure all vendors agree on implementation and 
expectation of
the symmetric hash function.

The way the Intel NICs are achieving this hash symmetry is by XORing 
the source and destination values of the IP and L4 ports and then 
feeding these values to the regular Toeplitz (in-tree) hash algorithm.

For example, for UDP/IPv4, the input fields for the Toeplitz hash 
would be:

(SRC_IP, DST_IP, SRC_PORT,  DST_PORT)

So you mangle the input. This is different than the paper you
referenced below which doesn't change the input but it modifies the RSS
algorithm and uses a special hash key.

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If symmetric Toeplitz is set, the NIC XOR the src and dst fields:

(SRC_IP^DST_IP ,  SRC_IP^DST_IP, SRC_PORT^DST_PORT, SRC_PORT^DST_PORT)

This way, the output hash would be the same for both flow directions. 
Same is applicable for IPv6, TCP and SCTP.

I understand the motivation, I just want to make sure the 
interpretation is
clear, I agree with Jakub, we should use a clear name for the ethtool
parameter or allow users to select "xor-ed"/"sorted" fields as Jakub
suggested.

quoted

Regarding the documentation, the above is available in our public 
datasheets [2]. In the final version, I can add similar explanation 
in the headers (kdoc) and under "Documentation/networking/" so that 
there is a clear understanding of the algorithm.

Please do define the behavior.

When I hear symmetric Toeplitz, my initial assumption was also
sorted fields, as implemented in __flow_hash_consistentify.

If this is something else, agreed that that is good to make
crystal clear in name and somewhere in the kernel Documentation.
xor-symmetric hash?