See also this commit.

A port of the Illumos Crypto Framework to a Linux kernel module (found in module/icp). This is needed to do the actual encryption work. We cannot use the Linux kernel’s built in crypto api because it is only exported to GPL-licensed modules. Having the ICP also means the crypto code can run on any of the other kernels under OpenZFS. I ended up porting over most of the internals of the framework, which means that porting over other API calls (if we need them) should be fairly easy. Specifically, I have ported over the API functions related to encryption, digests, macs, and crypto templates. The ICP is able to use assembly-accelerated encryption on amd64 machines and AES-NI instructions on Intel chips that support it. There are place-holder directories for similar assembly optimizations for other architectures (although they have not been written).