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Date: Fri, 28 Aug 2015 06:54:28 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Dave Hansen <dave@sr71.net>
Cc: dave.hansen@linux.intel.com, bp@alien8.de, fenghua.yu@intel.com,
        hpa@zytor.com, x86@kernel.org, tim.c.chen@linux.intel.com,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 09/11] x86, fpu: correct and check XSAVE xstate size
 calculations
Message-ID: <20150828045427.GB25556@gmail.com>
References: <20150827171102.1BDF27E5@viggo.jf.intel.com>
 <20150827171110.ADA00471@viggo.jf.intel.com>
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* Dave Hansen <dave@sr71.net> wrote:

> +static int xfeature_is_supervisor(int xfeature_nr)
> +{
> +	/*
> +	 * We currently do not suport supervisor states, but if
> +	 * we did, we could find out like this.
> +	 *
> +	 * SDM says: If state component i is a user state component,
> +	 * ECX[0] return 0; if state component i is a supervisor
> +	 * state component, ECX[0] returns 1.
> +	u32 eax, ebx, ecx, edx;
> +	cpuid_count(XSTATE_CPUID, xfeature_nr, &eax, &ebx, &ecx, &edx);
> +	return !!(ecx & 1);
> +	*/
> +	return 0;
> +}

So if this CPUID is documented to work, why not use it to sanity check things?

I.e. do something like:

	u32 eax, ebx, ecx, edx;

	cpuid_count(XSTATE_CPUID, xfeature_nr, &eax, &ebx, &ecx, &edx);

	/* Linux doesn't support supervisor states (yet): */
	WARN_ON_ONCE(ecx & 1);

	return 0;

That would give us a gentle way to double check our assumptions here.

Thanks,

	Ingo