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Subject: Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for
 __fpu__restore_sig()
From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     Borislav Petkov <bp@alien8.de>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Peter Zijlstra <peterz@infradead.org>
Date:   Fri, 28 Feb 2020 14:13:29 -0800
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On Fri, 2020-02-28 at 22:47 +0100, Borislav Petkov wrote:
> On Fri, Feb 28, 2020 at 01:22:39PM -0800, Yu-cheng Yu wrote:
> > The code is for sigreturn only.  Because of lazy-restore,
> > copy_xregs_to_kernel() does not happen all the time.
> 
> What does "not all the time" mean? You need to quantify this more
> precisely.

If the XSAVES buffer already has current data (i.e. TIF_NEED_FPU_LOAD is
set), then skip copy_xregs_to_kernel().  This happens when the task was
context-switched out and has not returned to user-mode.