linux-kernel - Re: [PATCH 17/37] x86, pkeys: add functions to fetch PKRU

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Date:	Mon, 30 Nov 2015 07:51:48 -0800
From:	Dave Hansen <dave@...1.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	dave.hansen@...ux.intel.com
Subject: Re: [PATCH 17/37] x86, pkeys: add functions to fetch PKRU

On 11/27/2015 01:51 AM, Thomas Gleixner wrote:
> On Mon, 16 Nov 2015, Dave Hansen wrote:
>> +#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
>> +static inline u32 __read_pkru(void)
>> +{
>> +	unsigned int eax, edx;
>> +	unsigned int ecx = 0;
>> +	unsigned int pkru;
>> +
>> +	asm volatile(".byte 0x0f,0x01,0xee\n\t"
>> +		     : "=a" (eax), "=d" (edx)
>> +		     : "c" (ecx));
>> +	pkru = eax;
>> +	return pkru;
> 
> Wouldn't a simple 'return eax;' be sufficient?

Yes.  I wanted to make it clear that EAX specifically was where the data
we want was ending up.  But, I guess the "=a" does that equally well.

I'll fix it up.

Also, looking at this, it also deserves a mention that this is the
"rdpkru" instruction.  Otherwise, it's hard to figure out what
instruction this is.

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