Re: [Xen-devel] libxc: Casting of xen virtual address type xen_vaddr_t to signed int64 type: (int64_t)vaddr

[Julien Grall <julien.grall@xxxxxxx>]

Hi Viktor,

Is there any specific reason I ended up to be CCed in a middle of an x86 thread?

Cheers,

On 20/05/2019 10:17, Viktor Mitin wrote:
> > > Mean that result of "(int64_t)vaddr >> 63" can be 0 or 1.
> > > So the next code may not work properly in case of another 'implementations'.
> > > With another compiler (i.e. clang, etc) this code may introduce bugs
> > > which are hard to find.
> > >
> > > ((int64_t)vaddr >> 47) == ((int64_t)vaddr >> 63)
> > >
> > > For this reason it is better to avoid implementation-defined code.
> > Well, ideally we'd like to get away without using implementation
> > defined behavior. But I'm afraid we're quite far from that, and
> > we'd not always be willing to accept the worse source and/or
> > binary code that would be needed to avoid it.
> How about using the next one-liner to avoid implementation-defined code :
>
> //Returns true in case when the top left 17 bits are all zero or are all one
> return ((!(vaddr >> 47)) || (((vaddr >> 47)&0x1FFFF) == 0x1FFFF))
>
> Agree?
>
> Thanks
--
Julien Grall

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