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Date: Fri, 28 Aug 2015 07:31:47 -0700
From: Dave Hansen <dave@sr71.net>
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To: Ingo Molnar <mingo@kernel.org>
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>,
        "Kleen, Andi" <andi.kleen@intel.com>
Subject: Re: [PATCH 09/11] x86, fpu: correct and check XSAVE xstate size calculations
References: <20150827171102.1BDF27E5@viggo.jf.intel.com> <20150827171110.ADA00471@viggo.jf.intel.com> <20150828045427.GB25556@gmail.com>
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On 08/27/2015 09:54 PM, Ingo Molnar wrote:
> 
> * 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.

Actually, the newest state that you will see in the wild is for
Processor Trace, and it _is_ a supervisor state.  However, we don't use
it in Linux for our Processor Trace support, and Andi says we probably
never will.

So we probably shouldn't warn on it.