Re: [patch 3/3] x86/process: Optimize TIF_NOTSC switch

[Andy Lutomirski]

On Fri, Dec 16, 2016 at 12:50 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Thu, 15 Dec 2016, Andy Lutomirski wrote:
>> On Thu, Dec 15, 2016 at 8:44 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.