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From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 16 Dec 2016 10:34:32 -0800
Message-ID: <CALCETrXZP-5xuOE0nWzAoa2FufMGS7bgKeQLdwS4eOhHhS225w@mail.gmail.com>
Subject: Re: [patch 3/3] x86/process: Optimize TIF_NOTSC switch
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Kyle Huey <khuey@kylehuey.com>,
        Andy Lutomirski <luto@kernel.org>
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On Fri, Dec 16, 2016 at 12:50 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Thu, 15 Dec 2016, Andy Lutomirski wrote:
>> On Thu, Dec 15, 2016 at 8:44 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>> > +static inline void cr4_toggle_bits(unsigned long mask)
>> > +{
>> > +       unsigned long cr4;
>> > +
>> > +       cr4 = this_cpu_read(cpu_tlbstate.cr4);
>> > +       cr4 ^= mask;
>> > +       this_cpu_write(cpu_tlbstate.cr4, cr4);
>> > +       __write_cr4(cr4);
>> > +}
>>
>> This scares me for the same reason as BTF, although this should at
>> least be less fragile.  But how about:
>
> If that is fragile then all cr4 manipulation code is fragile because it
> relies on cpu_tlbstate.cr4. The TIF flag and that per cpu thing are kept in
> sync.

True.