Re: Lazy FPU restoration / moving kernel_fpu_end() to context switch

From: Jason A. Donenfeld
Date: Fri Jun 15 2018 - 16:34:06 EST


On Fri, Jun 15, 2018 at 8:32 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> quite in the form you imagined. The idea that we've tossed around is
> to restore FPU state on return to user mode. Roughly, we'd introduce
> a new thread flag TIF_FPU_UNLOADED (name TBD).
> prepare_exit_to_usermode() would notice this flag, copy the fpstate to
> fpregs, and clear the flag. (Or maybe exit_to_usermode_loop() -- No
> one has quite thought it through, but I think it should be outside the
> loop.) We'd update all the FPU accessors to understand the flag.

Yes! This is exactly what I was thinking. Then those calls to begin()
and end() could be placed as close to the actual FPU usage as
possible.