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

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Date:   Tue, 14 Mar 2017 21:36:22 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kyle Huey <me@...ehuey.com>
cc:     Robert O'Callahan <robert@...llahan.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Len Brown <len.brown@...el.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        David Matlack <dmatlack@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
        user-mode-linux-devel@...ts.sourceforge.net,
        user-mode-linux-user@...ts.sourceforge.net,
        linux-kselftest@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v15 6/9] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID

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