From: Sebastian Andrzej Siewior > Sent: 03 April 2019 17:41 ... > To access the FPU registers in kernel we need: > - disable preemption to avoid that the scheduler switches tasks. By > doing so it would set TIF_NEED_FPU_LOAD and the FPU registers would be > not valid. > - disable BH because the softirq might use kernel_fpu_begin() and then > set TIF_NEED_FPU_LOAD instead loading the FPU registers on completion. Is there a possible optimisation here for kernel threads? Since there is no 'user FP state' the 'kernel FP state' can be saved by a task switch or softirq. You'd still want the kernel thread to bracket fpu usage (or at least say that it is a kernel thread that always needs the fpu) to avoid having to save and restore the fpu registers on every context switch to/from a kernel thread. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
