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Subject: Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows
 correct xstates copy from kernel to user directly
To: Yu-cheng Yu <yu-cheng.yu@intel.com>
References: <cover.1457038929.git.yu-cheng.yu@intel.com>
 <b4b6fd6f2b928474748abb3d842aef025cbd10da.1457038929.git.yu-cheng.yu@intel.com>
 <5723BF63.2000100@linux.intel.com> <20160429224338.GA15757@test-lenovo>
 <5723FE20.7000906@linux.intel.com> <20160502155734.GA21577@test-lenovo>
 <57277B11.9080802@linux.intel.com> <20160502163410.GA21734@test-lenovo>
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>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <572783C3.8040907@linux.intel.com>
Date: Mon, 2 May 2016 09:43:47 -0700
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On 05/02/2016 09:34 AM, Yu-cheng Yu wrote:
> On Mon, May 02, 2016 at 09:06:41AM -0700, Dave Hansen wrote:
>> On 05/02/2016 08:57 AM, Yu-cheng Yu wrote:
>>> On Fri, Apr 29, 2016 at 05:36:48PM -0700, Dave Hansen wrote:
>>>> If may_copy_fpregs_to_sigframe() were called from a slightly different
>>>> context, or if we change the call-site, what breaks?
>>>>
>>>> In other words. if we can still "may_copy_fpregs_to_sigframe()" no
>>>> matter the state of fpu.fpstate_active, then I don't think we should be
>>>> checking it in may_copy_fpregs_to_sigframe().
>>>
>>> Do you mean, don't check fpu.fpstate_active here?
>>
>> Not really.  I'm asking *why* the check is there.
> 
> 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.