Re: [PATCH v9 00/27] x86: load FPU registers on return to userland

Thomas Gleixner
2019-04-08T17:08:57Z

On Wed, 3 Apr 2019, Sebastian Andrzej Siewior wrote:

> This is a refurbished series originally started by by Rik van Riel. The
> goal is load the FPU registers on return to userland and not on every
> context switch. By this optimisation we can:
> - avoid loading the registers if the task stays in kernel and does
> not return to userland
> - make kernel_fpu_begin() cheaper: it only saves the registers on the
> first invocation. The second invocation does not need save them again.
>
> 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.

So aside of that one hunk in 24/27 which is either wrong or needs some
information in the changelog I couldn't find anything disturbing.

With that addressed:

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>