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Date: Mon, 2 May 2016 10:19:36 -0700
From: Yu-cheng Yu <yu-cheng.yu@intel.com>
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org, Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@suse.de>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>
Subject: Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows
 correct xstates copy from kernel to user directly
Message-ID: <20160502171936.GA22224@test-lenovo>
References: <cover.1457038929.git.yu-cheng.yu@intel.com>
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On Mon, May 02, 2016 at 09:43:47AM -0700, Dave Hansen wrote:
> > If (fpu.fpstate_active == 0), then the task does not use FPU; we don't
> > want to save these registers, right?  
> 
> No.  It's possible to have fpstate_active=0 while fpregs_active=1.  Such
> a task uses the FPU, but just hasn't done an XSAVE* to save the register
> content to the fpstate buffer.
> 
> Note, this is just theoretical, and does not happen in this particular
> call path today.

What about...

static int may_copy_fpregs_to_sigframe(void)
{
	if (fpregs_active())
		return 1;


	WARN_ONCE(!current->thread.fpu.fpstate_active,
		  "direct FPU save with no math use\n");

	if (boot_cpu_has(X86_FEATURE_XSAVES))
		return 1;

	return 0;
}