Subject: Re: [PATCH net-next] net: expand NETDEV_RSS_KEY_LEN to 256 bytes

From: Willem de Bruijn <[email-masked]>
Date: Thu, 22 Jan 2026 15:22:52 -0500

Eric Dumazet wrote:
> NETDEV_RSS_KEY_LEN has been set to 52 bytes in 2014, until now.
> 
> Jakub suggested we bump the size to 128 bytes or more.
> 
> Some drivers (like idpf) were already working around the core limit.

Idpf still bounds to the max?

Iff respinning it may be informative to explain how drivers intend to
use limits beyond standard Toeplitz. The tunneling that Jakub
mentioned. AFAIK there is no Linux control API to configure the device
RSS block in that way?

> 
> Since this change might cause some issues in admin scripts,
> bump it directly to 256 in one go.
> 
> tjbp26:~# cat /proc/sys/net/core/netdev_rss_key | wc -c
> 768
> 
> tjbp26:~# ethtool -x eth1
> RX flow hash indirection table for eth1 with 32 RX ring(s):
> ...
> RSS hash key:
> fe:16:5b:2f:93:85:c2:c9:c1:ef:bd:60:c6:e0:2b:99:4d:bf:b7:14:c8:1e:8d:cb:31:17:51:da:55:eb:91:d9:9e:f9:89:9b:44:a1:dc:08:72:3a:b3:d6:31:86:9a:fe:02:3a:0d:eb:a1:7c:f5:a3:51:3b:08:56:c9:3f:71:69:01:ba:70:38
> RSS hash function:
>     toeplitz: on
>     xor: off
>     crc32: off
> 
> Suggested-by: Jakub Kicinski <[email-masked]>
> Link: https://lore.kernel.org/netdev/[email-masked]/
> Signed-off-by: Eric Dumazet <[email-masked]>

Reviewed-by: Willem de Bruijn <[email-masked]>

> diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
> index 05dd55cf8b58e6c6fce498a11c09f23fd56d8f34..0f761d4b94471a5f8bbe0810922cf59e009be99d 100644
> --- a/net/core/sysctl_net_core.c
> +++ b/net/core/sysctl_net_core.c
> @@ -325,10 +325,16 @@ static int proc_do_dev_weight(const struct ctl_table *table, int write,
>  static int proc_do_rss_key(const struct ctl_table *table, int write,
>  			   void *buffer, size_t *lenp, loff_t *ppos)
>  {
> -	struct ctl_table fake_table;
>  	char buf[NETDEV_RSS_KEY_LEN * 3];
> +	struct ctl_table fake_table;
> +	char *pos = buf;
> +
> +	for (int i = 0; i < NETDEV_RSS_KEY_LEN; i++) {
> +		pos = hex_byte_pack(pos, netdev_rss_key[i]);
> +		*pos++ = ':';
> +	}
> +	*(--pos) = 0;
>  
> -	snprintf(buf, sizeof(buf), "%*phC", NETDEV_RSS_KEY_LEN, netdev_rss_key);

Why change the print method?

>  	fake_table.data = buf;
>  	fake_table.maxlen = sizeof(buf);
>  	return proc_dostring(&fake_table, write, buffer, lenp, ppos);
> -- 
> 2.52.0.457.g6b5491de43-goog
>