netdev - Re: [Intel-wired-lan] [PATCH net-next v4 1/6] net: ethtool: allow symmetric-xor RSS hash for any flow type

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Date: Tue, 31 Oct 2023 18:12:49 +0200
From: Gal Pressman <gal@...dia.com>
To: Ahmed Zaki <ahmed.zaki@...el.com>, Jakub Kicinski <kuba@...nel.org>,
 Alexander H Duyck <alexander.duyck@...il.com>
Cc: mkubecek@...e.cz, andrew@...n.ch, willemdebruijn.kernel@...il.com,
 Wojciech Drewek <wojciech.drewek@...el.com>, corbet@....net,
 netdev@...r.kernel.org, linux-doc@...r.kernel.org,
 jesse.brandeburg@...el.com, edumazet@...gle.com, anthony.l.nguyen@...el.com,
 horms@...nel.org, vladimir.oltean@....com,
 Jacob Keller <jacob.e.keller@...el.com>, intel-wired-lan@...ts.osuosl.org,
 pabeni@...hat.com, davem@...emloft.net
Subject: Re: [Intel-wired-lan] [PATCH net-next v4 1/6] net: ethtool: allow
 symmetric-xor RSS hash for any flow type

On 31/10/2023 17:14, Ahmed Zaki wrote:
> 
> 
> On 2023-10-31 08:45, Gal Pressman wrote:
>> On 31/10/2023 16:40, Ahmed Zaki wrote:
>>>
>>>
>>> On 2023-10-31 06:00, Gal Pressman wrote:
>>>> On 29/10/2023 18:59, Ahmed Zaki wrote:
>>>>>
>>>>>
>>>>> On 2023-10-29 06:48, Gal Pressman wrote:
>>>>>> On 29/10/2023 14:42, Ahmed Zaki wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 2023-10-29 06:25, Gal Pressman wrote:
>>>>>>>> On 21/10/2023 3:00, Ahmed Zaki wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 2023-10-20 17:49, Jakub Kicinski wrote:
>>>>>>>>>> On Fri, 20 Oct 2023 17:14:11 -0600 Ahmed Zaki wrote:
>>>>>>>>>>> I replied to that here:
>>>>>>>>>>>
>>>>>>>>>>> https://lore.kernel.org/all/afb4a06f-cfba-47ba-adb3-09bea7cb5f00@intel.com/
>>>>>>>>>>>
>>>>>>>>>>> I am kind of confused now so please bear with me. ethtool either
>>>>>>>>>>> sends
>>>>>>>>>>> "ethtool_rxfh" or "ethtool_rxnfc". AFAIK "ethtool_rxfh" is the
>>>>>>>>>>> interface
>>>>>>>>>>> for "ethtool -X" which is used to set the RSS algorithm. But we
>>>>>>>>>>> kind of
>>>>>>>>>>> agreed to go with "ethtool -U|-N" for symmetric-xor, and that
>>>>>>>>>>> uses
>>>>>>>>>>> "ethtool_rxnfc" (as implemented in this series).
>>>>>>>>>>
>>>>>>>>>> I have no strong preference. Sounds like Alex prefers to keep it
>>>>>>>>>> closer
>>>>>>>>>> to algo, which is "ethtool_rxfh".
>>>>>>>>>>
>>>>>>>>>>> Do you mean use "ethtool_rxfh" instead of "ethtool_rxnfc"? how
>>>>>>>>>>> would
>>>>>>>>>>> that work on the ethtool user interface?
>>>>>>>>>>
>>>>>>>>>> I don't know what you're asking of us. If you find the code to
>>>>>>>>>> confusing
>>>>>>>>>> maybe someone at Intel can help you :|
>>>>>>>>>
>>>>>>>>> The code is straightforward. I am confused by the requirements:
>>>>>>>>> don't
>>>>>>>>> add a new algorithm but use "ethtool_rxfh".
>>>>>>>>>
>>>>>>>>> I'll see if I can get more help, may be I am missing something.
>>>>>>>>>
>>>>>>>>
>>>>>>>> What was the decision here?
>>>>>>>> Is this going to be exposed through ethtool -N or -X?
>>>>>>>
>>>>>>> I am working on a new version that uses "ethtool_rxfh" to set the
>>>>>>> symmetric-xor. The user will set per-device via:
>>>>>>>
>>>>>>> ethtool -X eth0 hfunc toeplitz symmetric-xor
>>>>>>>
>>>>>>> then specify the per-flow type RSS fields as usual:
>>>>>>>
>>>>>>> ethtool -N|-U eth0 rx-flow-hash <flow_type> s|d|f|n
>>>>>>>
>>>>>>> The downside is that all flow-types will have to be either
>>>>>>> symmetric or
>>>>>>> asymmetric.
>>>>>>
>>>>>> Why are we making the interface less flexible than it can be with -N?
>>>>>
>>>>> Alexander Duyck prefers to implement the "symmetric-xor" interface
>>>>> as an
>>>>> algorithm or extension (please refer to previous messages), but
>>>>> ethtool
>>>>> does not provide flowtype/RSS fields setting via "-X". The above
>>>>> was the
>>>>> best solution that we (at Intel) could think of.
>>>>
>>>> OK, it's a weird we're deliberately limiting our interface, given
>>>> there's already hardware that supports controlling symmetric hashing
>>>> per
>>>> flow type.
>>>>
>>>> I saw you mentioned the way ice hardware implements symmetric-xor
>>>> somewhere, it definitely needs to be added somewhere in our
>>>> documentation to prevent confusion.
>>>> mlx5 hardware also does symmetric hashing with xor, but not exactly as
>>>> you described, we need the algorithm to be clear.
>>>
>>> Sure. I will add more ice-specific doc in:
>>> Documentation/networking/device_drivers/ethernet/intel/ice.rst
>>
>> I was thinking of somewhere more generic, where ethtool users (not
>> necessarily ice users) can refer to.
>>
>> Perhaps Documentation/networking/ethtool-netlink.rst? Or ethtool man
>> page?
> 
> Do you mean add vendor-specific implementation details to common docs?
> Not sure if I have seen this before. Any examples?
> 
> Or, we can add a note in ethtool doc that each vendor's implementation
> is different and "Refer to your vendor's specifications for more info".

It's a generic ethtool flag, its documentation shouldn't be vendor specific.
The documentation should reflect the exact details about the algorithm,
then other vendors can either use it, or add a new symmetric flag and
document it separately.