Re: [PATCH 2/3] x86, fpu: don't abuse ->has_fpu in __kernel_fpu_{begin,end}()

Rik van Riel
2015-01-16T02:27:26Z

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On 01/15/2015 02:20 PM, Oleg Nesterov wrote:
> Now that we have in_kernel_fpu we can remove
> __thread_clear_has_fpu() in __kernel_fpu_begin(). And this allows
> to replace the asymmetrical and nontrivial use_eager_fpu +
> tsk_used_math check in kernel_fpu_end() with the same
> __thread_has_fpu() check.
>
> The logic becomes really simple; if _begin() does save() then
> _end() needs restore(), this is controlled by __thread_has_fpu().
> Otherwise they do clts/stts unless use_eager_fpu().
>
> Not only this makes begin/end symmetrical and imo more
> understandable, potentially this allows to change irq_fpu_usable()
> to avoid all other checks except "in_kernel_fpu".
>
> Also, with this patch __kernel_fpu_end() does
> restore_fpu_checking() and WARNs if it fails instead of
> math_state_restore(). I think this looks better because we no
> longer need __thread_fpu_begin(), and it would be better to report
> the failure in this case.
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com> ---
> arch/x86/kernel/i387.c | 19 ++++++------------- 1 files changed,
> 6 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c index
> a815723..12088a3 100644 --- a/arch/x86/kernel/i387.c +++
> b/arch/x86/kernel/i387.c @@ -81,9 +81,7 @@ void
> __kernel_fpu_begin(void) this_cpu_write(in_kernel_fpu, true);
>
> if (__thread_has_fpu(me)) { - __thread_clear_has_fpu(me);

I will put that line back in the patch series that defers the
loading of FPU state until the switch to user space, but I
guess we can go either way for now...

> __save_init_fpu(me); - /* We do 'stts()' in __kernel_fpu_end() */
> } else if (!use_eager_fpu()) { this_cpu_write(fpu_owner_task,
> NULL); clts(); @@ -93,17 +91,12 @@
> EXPORT_SYMBOL(__kernel_fpu_begin);
>
> void __kernel_fpu_end(void) { - if (use_eager_fpu()) { - /* - *
> For eager fpu, most the time, tsk_used_math() is true. - *
> Restore the user math as we are done with the kernel usage. - *
> At few instances during thread exit, signal handling etc, - *
> tsk_used_math() is false. Those few places will take proper - *
> actions, so we don't need to restore the math here. - */ - if
> (likely(tsk_used_math(current))) - math_state_restore(); - } else
> { + struct task_struct *me = current; + + if (__thread_has_fpu(me))
> { + if (WARN_ON(restore_fpu_checking(me))) + drop_init_fpu(me);
> + } else if (!use_eager_fpu()) { stts(); }
>
>

- --
All rights reversed
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