Re: [Xen-devel] [PATCH] tools/libxc: make volatile keyword for bitmap operations optional

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 30/01/2012 09:10, "Olaf Hering" <olaf@xxxxxxxxx> wrote:

>>> tools/libxc: make volatile keyword for bitmap operations optional
>>> 
>>> Except for xc_save, all bitmaps maintained by xc_bitops.h are used in single
>>> threaded applications. So nothing will change the bitmaps content, adding
>>> volatile adds just unneeded memory reloads.
>> 
>> The bitops aren't threadsafe anyway, as none of them use atomic rmw
>> instructions. I suspect the volatile declarations are completely pointless
>> and can just be removed.
> 
> Will gcc use the right thing if the array is passed to the hyperviso,
> and will it reload everything after the hypercall? If yes, the volatile
> can indeed go.

Of course. Lots of things would fail to work if calls to outside the current
linkage unit didn't flush/invalidate cached shared-data accesses. That's a
fundamental C compiler thing.

 -- Keir



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