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

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On Thu, Dec 15, 2016 at 8:44 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> Provide and use a seperate helper for toggling the DEBUGCTLMSR_BTF bit
> instead of doing it open coded with a branch and eventually evaluating
> boot_cpu_data twice.
>
> x86_64:
> 3694       8505      16   12215    2fb7 Before
> 3662       8505      16   12183    2f97 After
>
> i386:
> 5986       9388    1804   17178    431a Before
> 5906       9388    1804   17098    42ca After
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>  arch/x86/include/asm/processor.h |   12 ++++++++++++
>  arch/x86/kernel/process.c        |   10 ++--------
>  2 files changed, 14 insertions(+), 8 deletions(-)
>
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -676,6 +676,18 @@ static inline void update_debugctlmsr(un
>         wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctlmsr);
>  }
>
> +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.

How about adding a real feature bit and doing:

if (!static_cpu_has(X86_FEATURE_BLOCKSTEP))
  return;

rdmsrl(MSR_IA32_DEBUGCTLMSR, msrval);
msrval &= DEBUGCTLMSR_BTF;
msrval |= (tifn >> TIF_BLOCKSTEP) << DEBUGCTLMSR_BIT;

--Andy

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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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