linux-kernel - Re: [RFC PATCH 08/11] x86,fpu: restore user FPU state lazily after __kernel_fpu

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Message-ID: <20150114184343.GA17640@redhat.com>
Date:	Wed, 14 Jan 2015 19:43:43 +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 08/11] x86,fpu: restore user FPU state lazily after
	__kernel_fpu_end

On 01/11, riel@...hat.com wrote:
>
> --- a/arch/x86/kernel/i387.c
> +++ b/arch/x86/kernel/i387.c
> @@ -89,13 +89,11 @@ 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.
> +		 * Make sure the user math state is restored on return to
> +		 * userspace.
>  		 */
>  		if (likely(tsk_used_math(current)))
> -			math_state_restore();
> +			set_thread_flag(TIF_LOAD_FPU);

Hmm. And this looks obviously wrong if interrupted_user_mode().

Oleg.

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