To say what tialaramex said in a different way, the OS needs to save all the user space registers to the stack when it interrupts a task, so it can do it's own thing. Floats are held in separate registers, so as an optimization it can skip saving those registers to the stack and just not use them (as long as it doesn't call any code that might use floats, and returns back to the same task afterwards). This turns out to be a substantial performance improvement, because there are lots of small syscalls that return quickly.

128 bit integers not working is a bit dumber, the compiler relies on some functions being implemented for them to work, and for whatever reason the people behind rust in linux just haven't implemented those functions (yet?). They use the normal (non floating point) registers though, so there is nothing stopping them from being implemented. https://github.com/Rust-for-Linux/linux/issues/11#issuecomme...