Re: [PATCH 2/6] x86: kaslr: move CPU flags out of cpucheck

H. Peter Anvin
2013-04-26T22:14:44Z

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));