question about lazy FPU restore

[Rik van Riel]

Hi Suresh,

I have a question about the lazy fpu restore code in
switch_fpu_prepare.  Specifically, about the case where
the old task did not use the FPU, and the new task's
FPU state is still in the cpu.

        } else {
                old->fpu_counter = 0;
                old->thread.fpu.last_cpu = ~0;
                if (fpu.preload) {
                        new->fpu_counter++;
                        if (!use_eager_fpu() && fpu_lazy_restore(new, cpu))
                                fpu.preload = 0;
                        else
                                prefetch(new->thread.fpu.state);
                        __thread_fpu_begin(new);
                }
        }

In this branch, we call fpu_lazy_restore, which
confirms that the CPU still has the new task's state
in it.

However, if we are in eager fpu mode, we still end up
calling restore_fpu_checking from switch_fpu_finish,
even if the new task's FPU state is still resident in
the CPU.

Is there a particular reason we do this?

Would it be possible to always set fpu.preload = 0,
call clts, and __thread_set_has_fpu if fpu_lazy_restore
returns true?

That would allow us to skip the loading of FPU state
when re-entering a process that went briefly idle, before
getting something else to do, a common occurrance in
message passing workloads.
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