Re: [PATCH v15 6/9] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID

[Thomas Gleixner]

On Sat, 11 Mar 2017, Kyle Huey wrote:
>  static void init_intel_misc_features(struct cpuinfo_x86 *c)
>  {
>       u64 msr;
>  
> +     if (rdmsrl_safe(MSR_MISC_FEATURES_ENABLES, &msr))
> +             return;
> +
> +     msr = 0;
> +     wrmsrl(MSR_MISC_FEATURES_ENABLES, msr);
> +     this_cpu_write(msr_misc_features_shadow, msr);
> +
>       if (!rdmsrl_safe(MSR_PLATFORM_INFO, &msr)) {
>               if (msr & MSR_PLATFORM_INFO_CPUID_FAULT)
>                       set_cpu_cap(c, X86_FEATURE_CPUID_FAULT);
>       }
>  
>       probe_xeon_phi_r3mwait(c);

The way you are doing it breaks the ring3 mwait feature because you
overwrite the R3MWAIT bit the first time you update the MSR on context
switch.

>  }

What you really want is to fixup the r3mwait part:

static void probe_xeon_phi_r3mwait(c)
{
        ....
        if (ring3mwait_disabled)
                return;

        set_cpu_cap(c, X86_FEATURE_RING3MWAIT);
        this_cpu_or(msr_misc_features_shadow,
                    1UL << MSR_MISC_FEATURE_ENABLES_RING3MWAIT_BIT);

        if (c == &boot_cpu_data)
                ELF_HWCAP2 |= HWCAP2_RING3MWAIT;
}

and have a function for cpuid fault:

static void init_cpuid_fault(c)
{
        if (rdmsrl_safe(MSR_PLATFORM_INFO, &msr))
                return;
        if ((msr & MSR_PLATFORM_INFO_CPUID_FAULT))
                set_cpu_cap(c, X86_FEATURE_CPUID_FAULT);
}

and then do:

static void init_intel_misc_features(struct cpuinfo_x86 *c)
{
        u64 msr;
  
        if (rdmsrl_safe(MSR_MISC_FEATURES_ENABLES, &msr))
                return;

        /* Clear all MISC features */
        this_cpu_write(msr_misc_features_shadow, 0);

        /* Check the features and update the shadow control bits */
        init_cpuid_fault(c);
        probe_xeon_phi_r3mwait(c);

        msr_write(MSR_MISC_FEATURE_ENABLES,
                  this_cpu_read(msr_misc_features_shadow);
}

Hmm?

Thanks,

        tglx