The chacha-p10-crypto module provides optimised chacha routines for
Power10. It also selects CRYPTO_ARCH_HAVE_LIB_CHACHA which says it
provides chacha_crypt_arch() to generic code.

Notably the module needs to provide chacha_crypt_arch() regardless of
whether it is loaded on Power10 or an older CPU.

The implementation of chacha_crypt_arch() already has a fallback to
chacha_crypt_generic(), however the module as a whole fails to load on
pre-Power10, because of the use of module_cpu_feature_match().

This breaks for example loading wireguard:

  jostaberry-1:~ # modprobe -v wireguard
  insmod /lib/modules/6.8.0-lp155.8.g7e0e887-default/kernel/arch/powerpc/crypto/chacha-p10-crypto.ko.zst
  modprobe: ERROR: could not insert 'wireguard': No such device

Fix it by removing module_cpu_feature_match(), and instead check the
CPU feature manually. If the CPU feature is not found, the module
still loads successfully, but doesn't register the Power10 specific
algorithms. That allows chacha_crypt_generic() to remain available for
use, fixing the problem.

  [root@fedora ~]# modprobe -v wireguard
  insmod /lib/modules/6.8.0-00001-g786a790c4d79/kernel/net/ipv4/udp_tunnel.ko
  insmod /lib/modules/6.8.0-00001-g786a790c4d79/kernel/net/ipv6/ip6_udp_tunnel.ko
  insmod /lib/modules/6.8.0-00001-g786a790c4d79/kernel/lib/crypto/libchacha.ko
  insmod /lib/modules/6.8.0-00001-g786a790c4d79/kernel/arch/powerpc/crypto/chacha-p10-crypto.ko
  insmod /lib/modules/6.8.0-00001-g786a790c4d79/kernel/lib/crypto/libchacha20poly1305.ko
  insmod /lib/modules/6.8.0-00001-g786a790c4d79/kernel/drivers/net/wireguard/wireguard.ko
  [   18.910452][  T721] wireguard: allowedips self-tests: pass
  [   18.914999][  T721] wireguard: nonce counter self-tests: pass
  [   19.029066][  T721] wireguard: ratelimiter self-tests: pass
  [   19.029257][  T721] wireguard: WireGuard 1.0.0 loaded. See www.wireguard.com for information.
  [   19.029361][  T721] wireguard: Copyright (C) 2015-2019 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
Reported-by: Michal Suchánek <msuchanek@suse.de>
Closes: https://lore.kernel.org/all/20240315122005.GG20665@kitsune.suse.cz/Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240328130200.3041687-1-mpe@ellerman.id.au