From mboxrd@z Thu Jan  1 00:00:00 1970
From: Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH 17/22] x86/fpu: Prepare copy_fpstate_to_sigframe() for
 TIF_NEED_FPU_LOAD
Date: Wed, 30 Jan 2019 12:56:14 +0100
Message-ID: <20190130115614.GF18383@zn.tnic>
References: <20190109114744.10936-1-bigeasy@linutronix.de>
 <20190109114744.10936-18-bigeasy@linutronix.de>
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Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        kvm@vger.kernel.org, "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Rik van Riel <riel@surriel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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On Wed, Jan 09, 2019 at 12:47:39PM +0100, Sebastian Andrzej Siewior wrote:
> From: Rik van Riel <riel@surriel.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@surriel.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.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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