From: Philip Guenther <guenther@gmail.com>
Subject: Re: amd64/fpu: Avoid multiple FPU resets
To: Christian Ludwig <cludwig@genua.de>
Cc: tech@openbsd.org
Date: Fri, 13 Jun 2025 11:37:03 -0700

On Fri, Jun 13, 2025 at 5:32 AM Christian Ludwig <cludwig@genua.de> 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?

Philip Guenther