Message-ID: <517AFC38.5090500@zytor.com>
Date: Fri, 26 Apr 2013 15:14:16 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130311 Thunderbird/17.0.4
MIME-Version: 1.0
To: Kees Cook <keescook@chromium.org>
CC: linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        x86@kernel.org, Jarkko Sakkinen <jarkko.sakkinen@intel.com>,
        Matthew Garrett <mjg@redhat.com>,
        Matt Fleming <matt.fleming@intel.com>,
        Eric Northup <digitaleric@google.com>,
        Dan Rosenberg <drosenberg@vsecurity.com>,
        Julien Tinnes <jln@google.com>, Will Drewry <wad@chromium.org>
Subject: Re: [PATCH 2/6] x86: kaslr: move CPU flags out of cpucheck
References: <1367003005-5560-1-git-send-email-keescook@chromium.org> <1367003005-5560-3-git-send-email-keescook@chromium.org> <517AF5D6.1040800@zytor.com>
In-Reply-To: <517AF5D6.1040800@zytor.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1267
Lines: 47

On 04/26/2013 02:47 PM, H. Peter Anvin wrote:
> On 04/26/2013 12:03 PM, Kees Cook wrote:
>> +
>> +static inline void cpuid(u32 id, u32 *a, u32 *b, u32 *c, u32 *d)
>> +{
>> +	/* Handle x86_32 PIC using ebx. */
>> +	asm volatile("movl %%ebx, %%edi	\n\t"
>> +		     "cpuid		\n\t"
>> +		     "xchgl %%edi, %%ebx\n\t"
>> +		    : "=a" (*a),
>> +		      "=D" (*b),
>> +		      "=c" (*c),
>> +		      "=d" (*d)
>> +		    : "a" (id)
>> +	);
>> +}
> 
> Please don't constrain registers unnecessarily.
> 
> You can use "=r" there and let gcc assign whatever free register it pleases.
> 
> You can also limit that to only:
> 
> #if defined(__i386__) && defined(__PIC__)
> 

How is this for a "beauty":


#if defined(__i386__) && defined (__PIC__)
# define EBX_REG "=r"
#else
# define EBX_REG "=b"
#endif

  asm volatile(".ifnc %%ebx,%3 ; movl %%ebx,%3 ; .endif ; "
	       "cpuid ; "
	       ".ifnc %%ebx,%3 ; xchgl %%ebx,%3 ; .endif"
	       : "=a" (*a), "=c" (*c), "=d" (*d),
		 EBX_REG (*b)
	       : "a" (leaf), "c" (subleaf));

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/