TCP-AO, similarly to TCP-MD5, needs to allocate tfms on a slow-path,
which is setsockopt() and use crypto ahash requests on fast paths,
which are RX/TX softirqs. Also, it needs a temporary/scratch buffer
for preparing the hash.

Rework tcp_md5sig_pool in order to support other hashing algorithms
than MD5. It will make it possible to share pre-allocated crypto_ahash
descriptors and scratch area between all TCP hash users.

Internally tcp_sigpool calls crypto_clone_ahash() API over pre-allocated
crypto ahash tfm. Kudos to Herbert, who provided this new crypto API.

I was a little concerned over GFP_ATOMIC allocations of ahash and
crypto_request in RX/TX (see tcp_sigpool_start()), so I benchmarked both
"backends" with different algorithms, using patched version of iperf3[2].
On my laptop with i7-7600U @ 2.80GHz:

                         clone-tfm                per-CPU-requests
TCP-MD5                  2.25 Gbits/sec           2.30 Gbits/sec
TCP-AO(hmac(sha1))       2.53 Gbits/sec           2.54 Gbits/sec
TCP-AO(hmac(sha512))     1.67 Gbits/sec           1.64 Gbits/sec
TCP-AO(hmac(sha384))     1.77 Gbits/sec           1.80 Gbits/sec
TCP-AO(hmac(sha224))     1.29 Gbits/sec           1.30 Gbits/sec
TCP-AO(hmac(sha3-512))    481 Mbits/sec            480 Mbits/sec
TCP-AO(hmac(md5))        2.07 Gbits/sec           2.12 Gbits/sec
TCP-AO(hmac(rmd160))     1.01 Gbits/sec            995 Mbits/sec
TCP-AO(cmac(aes128))     [not supporetd yet]      2.11 Gbits/sec

So, it seems that my concerns don't have strong grounds and per-CPU
crypto_request allocation can be dropped/removed from tcp_sigpool once
ciphers get crypto_clone_ahash() support.

[1]: https://lore.kernel.org/all/ZDefxOq6Ax0JeTRH@gondor.apana.org.au/T/#u
[2]: https://github.com/0x7f454c46/iperf/tree/tcp-md5-aoSigned-off-by: Dmitry Safonov <dima@arista.com>
Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>