From: "Theo de Raadt" Subject: Re: amd64/fpu: Avoid multiple FPU resets To: Philip Guenther Cc: Christian Ludwig , tech@openbsd.org Date: Fri, 13 Jun 2025 13:08:04 -0600 Philip Guenther wrote: > On Fri, Jun 13, 2025 at 5:32 AM Christian Ludwig wrote: > > All kernel crypto code follows the scheme: > > > > for (objects) { > > fpu_kernel_enter(); > > ... > > fpu_kernel_exit(); > > } > > > > In every iteration, fpu_kernel_exit() resets the FPU state and > > fpu_kernel_enter() resets it, again. FPU resets are expensive on some > > platforms. Doing the operation twice per loop iteration is clearly not > > necessary. > > > > The FPU is always in one of two states when we reach fpu_kernel_enter(). > > It either holds user state when CPUPF_USERXSTATE is set or it is in > > reset state already. The context switching code and signal code follow > > this assumption, too. So we can simply drop resetting the FPU in > > fpu_kernel_enter() when it does not hold user state. > > Hmm, yes, all the places that clear CPUF_USERXSTATE reset the state. > > Does fpu_kernel_enter() get used from interrupt context? Do we have > to worry about an interrupt occurring between the clearing of the flag > and the resetting of the state? Is that a remaining artifact of the old lazy code? Or is it just being careful? Should it assert, to identify a missing fpu_kernel_exit(), or are such bugs no longer a concern?