Re: [Xen-devel] [PATCH LINUX v5] xen: event channel arrays are xen_ulong_t and not unsigned long

[Will Deacon <will.deacon@xxxxxxx>]

Hi Ian,

On Tue, Mar 05, 2013 at 09:29:57AM +0000, Ian Campbell wrote:
> On Tue, 2013-03-05 at 08:08 +0000, Will Deacon wrote:
> > Cheers Rob, that was enough to reproduce for me. The problem is likely that
> > CONFIG_AEABI=n, so the ABI doesn't actually mandate even base registers for
> > 64-bit values in registers.
> 
> Me too.
> 
> > Ian -- this would be fixed if you used our atomic64 routines instead of
> > inventing your own :)
> 
> I looked and couldn't see an existing 64 bit xchg, was I looking in the
> wrong place? Ah, wait, I see atomic64_xchg now. But that needs an
> atomic64_t while I have a xen_ulong_t (which == 64 bits on ARM). This is
> a kernel<->hypervisor ABI so I can't just change it to an atomic64_t. I
> suppose I could cast (see below, untested) but that seems rather icky.

You can play some container_of tricks, like we do in cmpxchg64 to get this
right. Alternatively, we could look at an xchg8 implementation which some
other architectures have (although they seem to be 64-bit machines).

Will

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