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

David Laight
2019-04-04T14:00:42Z

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

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