Merge branch 'net-ethtool-make-netdev_rss_key_fill-spread-flows-over-all-queues'

Eric Dumazet says:

====================
net: ethtool: make netdev_rss_key_fill() spread flows over all queues

netdev_rss_key_fill() hands drivers a uniformly random key. Because the
Toeplitz hash is linear over GF(2), a random key is singular for a given
header field and queue count with probability 1/2: flows differing only
in the low order bits of that field (such as a burst of consecutive
ephemeral ports) then cannot reach all RX queues. On one affected 16-queue
host, only 4 queues received traffic until the key was replaced.

Patch 1 synchronizes proc_do_rss_key() with netdev_rss_key_fill() using an
smp_wmb()/smp_rmb() pair and a boolean flag so readers of
/proc/sys/net/core/netdev_rss_key print zero bytes until the key is fully
populated.

Patch 2 keeps the key random but constrains 1008 of its 2048 bits so that,
at every 16-bit aligned position and for every power-of-two queue count up
to 256, consecutive values of a field ending there visit every queue once.
Applying the fixup on the 16-bit grid covers standard 2-tuple/4-tuple
hashes as well as encapsulated or bitmap-selected layouts (such as PSP
inner TCP ports at byte 78) without enumerating them in the core. The
fixup is a bijection onto the set of valid keys, and redraws in the ~1 in
5 case where two 32-bit windows a multiple of 8 bits apart alias.

Patch 3 adds RSS key, indirection table, and flow-hash reporting to
netdevsim so the selftest can exercise both the host key and ethtool
netlink RSS reporting in a virtual machine without hardware.

Patch 4 adds a selftest under tools/testing/selftests/drivers/net/rss_key.py
checking /proc/sys/net/core/netdev_rss_key and the live RSS key,
indirection table, and flow-hash layouts reported by a device via ethtool
netlink.
====================

Link: https://patch.msgid.link/20260922163458.3900996-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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