I think the issue is the kernel is extremely concerned with the efficiency of the syscall path.
Very legitimately some benchmarks just measure that one path to see how many thousands of syscalls per second can be made.
To accelerate that path as much as possible, the linux kenel chooses not to incur the overhead of preserving the FP registers on every syscall.
So kernel code that uses FP must first ensure any registers it uses are preserved. I don't recall ever writing any FP kernel code, so I don't know what facilities are available to do that.
Greg
Greg I had a vague idea about overhead being incurred due to the mode switch, but your clear explanation makes it a lot easier to understand _where_ this happens. It's very helpful, thank you for your response. Julie