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Re: [patch 2/3] x86/process: Optimize TIF_BLOCKSTEP switch

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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 toggle_debugctlmsr(unsigned long mask)
> > +{
> > +       unsigned long msrval;
> > +
> > +#ifndef CONFIG_X86_DEBUGCTLMSR
> > +       if (boot_cpu_data.x86 < 6)
> > +               return;
> > +#endif
> > +       rdmsrl(MSR_IA32_DEBUGCTLMSR, msrval);
> > +       wrmsrl(MSR_IA32_DEBUGCTLMSR, msrval ^ mask);
> > +}
> > +
> 
> This scares me.  If the MSR ever gets out of sync with the TI flag,
> this will malfunction.  And IIRC the MSR is highly magical and the CPU
> clears it all by itself under a variety of not-so-well documented
> circumstances.

If that is true, then the code today is broken as well, when the flag has
been cleared and both prev and next have the flag set. Then it won't be
updated for the next task.

The we should not use the TIF flag and store a debugmask in thread info and
do:

	if (prev->debugmask || next->debugmask) {
    		if (static_cpu_has(X86_FEATURE_BLOCKSTEP)) {
			rdmsrl(MSR_IA32_DEBUGCTLMSR, msrval);
			msrval &= DEBUGCTLMSR_BTF;
			msrval |= next->debugmask;
		}
	}

Thanks,

	tglx

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[patch 0/3] x86/process: Optimize __switch_to_extra() Thomas Gleixner <tglx@linutronix.de> - 2016-12-15 17:50 +0100
  [patch 2/3] x86/process: Optimize TIF_BLOCKSTEP switch Thomas Gleixner <tglx@linutronix.de> - 2016-12-15 17:50 +0100
    Re: [patch 2/3] x86/process: Optimize TIF_BLOCKSTEP switch Andy Lutomirski <luto@amacapital.net> - 2016-12-15 18:30 +0100
      Re: [patch 2/3] x86/process: Optimize TIF_BLOCKSTEP switch Thomas Gleixner <tglx@linutronix.de> - 2016-12-16 10:00 +0100
        Re: [patch 2/3] x86/process: Optimize TIF_BLOCKSTEP switch Andy Lutomirski <luto@amacapital.net> - 2016-12-16 20:30 +0100
  [patch 1/3] x86/process: Optimize TIF checks in switch_to_extra() Thomas Gleixner <tglx@linutronix.de> - 2016-12-15 17:50 +0100
    Re: [patch 1/3] x86/process: Optimize TIF checks in  switch_to_extra() Thomas Gleixner <tglx@linutronix.de> - 2016-12-15 18:30 +0100
      Re: [patch 1/3] x86/process: Optimize TIF checks in switch_to_extra() Peter Zijlstra <peterz@infradead.org> - 2016-12-15 18:40 +0100
    Re: [patch 1/3] x86/process: Optimize TIF checks in switch_to_extra() Andy Lutomirski <luto@amacapital.net> - 2016-12-15 18:30 +0100
    Re: [patch 1/3] x86/process: Optimize TIF checks in switch_to_extra() Peter Zijlstra <peterz@infradead.org> - 2016-12-15 18:30 +0100

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