linux-kernel - Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for __fpu__restore

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Date:   Fri, 28 Feb 2020 14:13:29 -0800
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for
 __fpu__restore_sig()

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.