linux-kernel - [PATCH RFC v3 2/9] virtio_net: Add functions for hashing

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Message-ID: <20240916080137.508-1-gur.stavi@huawei.com>
Date: Mon, 16 Sep 2024 11:01:36 +0300
From: <gur.stavi@...wei.com>
To: <gur.stavi@...wei.com>
CC: <akihiko.odaki@...nix.com>, <andrew@...nix.com>, <corbet@....net>,
	<davem@...emloft.net>, <edumazet@...gle.com>, <jasowang@...hat.com>,
	<kuba@...nel.org>, <kvm@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
	<mst@...hat.com>, <netdev@...r.kernel.org>, <pabeni@...hat.com>,
	<shuah@...nel.org>, <virtualization@...ts.linux-foundation.org>,
	<willemdebruijn.kernel@...il.com>, <xuanzhuo@...ux.alibaba.com>,
	<yuri.benditovich@...nix.com>
Subject: [PATCH RFC v3 2/9] virtio_net: Add functions for hashing

> +
> +static inline void virtio_net_toeplitz(struct virtio_net_toeplitz_state *state,
> +				       const __be32 *input, size_t len)
> 
> The function calculates a hash value but its name does not make it
> clear. Consider adding a 'calc'.
> 
> +{
> +	u32 key;
> +
> +	while (len) {
> +		state->key++;
> +		key = be32_to_cpu(*state->key);
> 
> You perform be32_to_cpu to support both CPU endianities.
> If you will follow with an unconditional swab32, you could run the
> following loop on a more natural 0 to 31 always referring to bit 0
> and avoiding !!(key & bit):
> 
> key = swab32(be32_to_cpu(*state->key));
> for (i = 0; i < 32; i++, key >>= 1) {
> 	if (be32_to_cpu(*input) & 1)
> 		state->hash ^= state->key_buffer;
> 	state->key_buffer = (state->key_buffer << 1) | (key & 1);
> }
> 

Fixing myself, in previous version 'input' was tested against same bit.
Advantage is less clear now, replacing !! with extra shift.
However, since little endian CPUs are more common, the combination of
swab32(be32_to_cpu(x) will actually become a nop.
Similar tactic may be applied to 'input' by assigning it to local
variable. This may produce more efficient version but not necessary
easier to understand.

key = bswap32(be32_to_cpu(*state->key));
for (u32 bit = BIT(31); bit; bit >>= 1, key >>= 1) {
	if (be32_to_cpu(*input) & bit)
		state->hash ^= state->key_buffer;
	state->key_buffer =
		(state->key_buffer << 1) | (key & 1);
}

> 
> +
> +		for (u32 bit = BIT(31); bit; bit >>= 1) {
> +			if (be32_to_cpu(*input) & bit)
> +				state->hash ^= state->key_buffer;
> +
> +			state->key_buffer =
> +				(state->key_buffer << 1) | !!(key & bit);
> +		}
> +
> +		input++;
> +		len--;
> +	}
> +}
> +