linux-kernel - Re: [PATCH 17/22] x86/fpu: Prepare copy_fpstate_to_sigframe() for TIF_NEED_FPU

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Date:   Wed, 30 Jan 2019 12:56:14 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 17/22] x86/fpu: Prepare copy_fpstate_to_sigframe() for
 TIF_NEED_FPU_LOAD

On Wed, Jan 09, 2019 at 12:47:39PM +0100, Sebastian Andrzej Siewior wrote:
> From: Rik van Riel <riel@...riel.com>
> 
> The FPU registers need only to be saved if TIF_NEED_FPU_LOAD is not set.
> Otherwise this has been already done and can be skipped.
> 
> Signed-off-by: Rik van Riel <riel@...riel.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  arch/x86/kernel/fpu/signal.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index bf4e6caad305e..a25be217f9a2c 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -156,7 +156,16 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
>  			sizeof(struct user_i387_ia32_struct), NULL,
>  			(struct _fpstate_32 __user *) buf) ? -1 : 1;
>  
> -	copy_fpregs_to_fpstate(fpu);
> +	__fpregs_changes_begin();
> +	/*
> +	 * If we do not need to load the FPU registers at return to userspace
> +	 * then the CPU has the current state and we need to save it. Otherwise
> +	 * it is already done and we can skip it.
> +	 */
> +	if (!test_thread_flag(TIF_NEED_FPU_LOAD))
> +		copy_fpregs_to_fpstate(fpu);

I wonder if this flag would make the code more easy to follow by calling
it

	TIF_FPU_REGS_VALID

instead, to denote that the FPU registers in the CPU have a valid
content.

Then the test becomes:

	if (test_thread_flag(TIF_FPU_REGS_VALID))
		copy_fpregs_to_fpstate(fpu);

-- 
Regards/Gruss,
    Boris.

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