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Message-ID: <20231017173448.3f1c35aa@kernel.org> Date: Tue, 17 Oct 2023 17:34:48 -0700 From: Jakub Kicinski <kuba@...nel.org> To: Alexander Duyck <alexander.duyck@...il.com> Cc: Ahmed Zaki <ahmed.zaki@...el.com>, netdev@...r.kernel.org, intel-wired-lan@...ts.osuosl.org, corbet@....net, jesse.brandeburg@...el.com, anthony.l.nguyen@...el.com, davem@...emloft.net, edumazet@...gle.com, 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 Tue, 17 Oct 2023 13:41:18 -0700 Alexander Duyck wrote: > I am thinking of this from a software engineering perspective. This > symmetric-xor aka simplified-toeplitz is actually much cheaper to > implement in software than the original. As such I would want it to be > considered a separate algorithm as I could make use of something like > that when having to implement RSS in QEMU for instance. That's exactly why XOR and CRC32 _algorithms_ already exist. CPUs have instructions to do them word at a time. ETH_RSS_HASH_TOP_BIT, /* Configurable RSS hash function - Toeplitz */ ETH_RSS_HASH_XOR_BIT, /* Configurable RSS hash function - Xor */ ETH_RSS_HASH_CRC32_BIT, /* Configurable RSS hash function - Crc32 */ If efficient SW implementation is important why do some weird bastardized para-toeplitz and not crc32? Hashes fairly well from what I recall with the older NFPs. x86 has an instruction for it, IIRC it was part of SSE but on normal registers. > Based on earlier comments it doesn't change the inputs, it just > changes how I have to handle the data and the key. It starts reducing > things down to something like the Intel implementation of Flow > Director in terms of how the key gets generated and hashed. About Flow Director I know only that it is bad :)
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