Re: [Xen-devel] x86: fix rdrand asm()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.09.13 at 17:52, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 25/09/13 16:46, Jan Beulich wrote:
>> Just learned the hard way that at least for non-volatile asm()s gcc
>> indeed does what the documentation says: It may move it across jumps
>> (i.e. ahead of the cpu_has() check). While the documentation claims
>> that this can also happen for volatile asm()s, if that was the case
>> we'd have many more problems in our code (and e,g, Linux would too).
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
>> --- a/xen/include/asm-x86/random.h
>> +++ b/xen/include/asm-x86/random.h
>> @@ -8,7 +8,7 @@ static inline unsigned int arch_get_rand
>>      unsigned int val = 0;
>>  
>>      if ( cpu_has(&current_cpu_data, X86_FEATURE_RDRAND) )
>> -        asm ( ".byte 0x0f,0xc7,0xf0" : "+a" (val) );
>> +        __asm__ __volatile__ ( ".byte 0x0f,0xc7,0xf0" : "+a" (val) );
> 
> Any reason for using the double underscore versions?  They have
> identical meanings, and the prevailing style does appear to be without.

In header I prefer to use the name space safe variant with
the underscores, while in .c files I'd use the ones without. From
a strict POV that ought to be necessary for public headers only
(where we don't have any asm()s anyway), but I'd like to do it
this way as long as we're not really consistent with this
throughout the tree.

Jan


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