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 13:51:52 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>, 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>,
        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 2020-02-28 13:17:24 [+0100], Borislav Petkov wrote:
> On Thu, Feb 27, 2020 at 02:52:12PM -0800, Yu-cheng Yu wrote:
> > If TIF_NEED_FPU_LOAD is set, then xstates are already in the xsave buffer. 
> > We can skip saving them again.
> 
> Ok, then pls use test_and_set_thread_flag().

I've been told not to do this while I crafted kernel_fpu_begin() because
this would introduce an atomic operation which we want avoid.

Sebastian