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

Arnd Bergmann
2010-04-23T12:46:11Z

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.

Arnd