Re: [PATCH RFC v2 6/6] KVM: introduce a new API for getting dirty bitmaps

On 23.04.2010, at 14:53, Avi Kivity wrote:

> On 04/23/2010 03:46 PM, Arnd Bergmann wrote:
>> On Friday 23 April 2010, Avi Kivity wrote:
>>   
>>> On 04/23/2010 03:27 PM, Arnd Bergmann wrote:
>>>     
>>>>       
>>>>> Using a 64-bit integer avoids the problem (though perhaps not sufficient
>>>>> for s390, Arnd?)
>>>>> 
>>>>>         
>>>> When there is only a __u64 for the address, it will work on s390 as well,
>>>> gcc is smart enough to clear the upper bit on a cast from long to pointer.
>>>> 
>>>>       
>>> Ah, much more convenient than compat_ioctl.  I assume it part of the
>>> ABI, not a gccism?
>>>     
>> I don't think it's part of the ABI, but it's required to guarantee
>> that code like this works:
>> 
>> int compare_pointer(void *a, void *b)
>> {
>> 	unsigned long ai = (unsigned long)a, bi = (unsigned long)b;
>> 
>> 	return ai = bi; /* true if a and b point to the same object */
>> }
>> 
>> We certainly rely on this already.
>>   
> 
> Ah so the 31st bit is optional as far as userspace is concerned?  What does it mean? (just curious)

The 0x80000000 bit declares that a pointer is in 24-bit mode, so that applications can use the spare upper bits for random data.

See http://en.wikipedia.org/wiki/31-bit for an explanation.

Alex