linux-kernel - Re: [RFC PATCH 04/11] x86,fpu: defer FPU restore until return to userspace

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Message-ID: <20150113175804.GA29851@redhat.com>
Date:	Tue, 13 Jan 2015 18:58:04 +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 04/11] x86,fpu: defer FPU restore until return to
	userspace

On 01/11, riel@...hat.com wrote:
>
> Defer restoring the FPU state, if so desired, until the task returns to
> userspace.

And yet another concern ;) Although I feel that I am totally confused and
probably wrong.

> --- a/arch/x86/include/asm/fpu-internal.h
> +++ b/arch/x86/include/asm/fpu-internal.h
> @@ -382,6 +382,7 @@ static inline void drop_init_fpu(struct task_struct *tsk)
>  		else
>  			fxrstor_checking(&init_xstate_buf->i387);
>  	}
> +	clear_thread_flag(TIF_LOAD_FPU);
>  }

OK, but shouldn't (say) restore_user_xstate() clear TIF_LOAD_FPU too?
Otherwise, can't switch_fpu_finish() restore the wrong context later?

Or. Perhaps switch_fpu_finish() should do nothing if fpu.has_fpu == T,
I dunno.

Oleg.

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