linux-kernel - Re: [RFC PATCH 03/11] x86,fpu: move __thread_fpu

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Date:	Tue, 13 Jan 2015 16:24:30 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	riel@...hat.com
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	matt.fleming@...el.com, bp@...e.de, pbonzini@...hat.com,
	tglx@...utronix.de, luto@...capital.net
Subject: Re: [RFC PATCH 03/11] x86,fpu: move __thread_fpu_begin to when the
	task has the fpu

On 01/11, riel@...hat.com wrote:
>
> --- a/arch/x86/include/asm/fpu-internal.h
> +++ b/arch/x86/include/asm/fpu-internal.h
> @@ -420,7 +420,6 @@ static inline void switch_fpu_prepare(struct task_struct *old, struct task_struc
>  		if (preload) {
>  			new->thread.fpu_counter++;
>  			set_thread_flag(TIF_LOAD_FPU);
> -			__thread_set_has_fpu(new);
>  			prefetch(new->thread.fpu.state);
>  		} else if (!use_eager_fpu())
>  			stts();
> @@ -436,7 +435,6 @@ static inline void switch_fpu_prepare(struct task_struct *old, struct task_struc
>  				prefetch(new->thread.fpu.state);
>  				set_thread_flag(TIF_LOAD_FPU);
>  			}
> -			__thread_fpu_begin(new);
>  		}
>  		/* else: CR0.TS is still set from a previous FPU switch */
>  	}
> @@ -451,6 +449,7 @@ static inline void switch_fpu_prepare(struct task_struct *old, struct task_struc
>  static inline void switch_fpu_finish(struct task_struct *new)
>  {
>  	if (test_and_clear_thread_flag(TIF_LOAD_FPU)) {
> +		__thread_fpu_begin(new);
>  		if (unlikely(restore_fpu_checking(new)))
>  			drop_init_fpu(new);
>  	}

Then perhaps it makes sense to move fpu_lazy_restore() to fpu_finish() too ?

Either way, afaics we do not need use_eager_fpu() before fpu_lazy_restore(),
and this reminds me that every use_eager_fpu() check in switch_fpu_prepare()
looks confusing.

Oleg.

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