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

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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.

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