Re: [RFC PATCH v2 10/15] xen/arm64: port Linux's arm64 cmpxchg.h to Xen

[Julien Grall <julien@xxxxxxx>]

Hi Ash,

On 11/11/2020 21:51, Ash Wilding wrote:
> From: Ash Wilding <ash.j.wilding@xxxxxxxxx>
>
>   - s/arch_xchg/xchg/
>
>   - s/arch_cmpxchg/cmpxchg/
>
>   - Replace calls to BUILD_BUG() with calls to __bad_cmpxchg() as we
>     don't currently have a BUILD_BUG() macro in Xen and this will
>     equivalently cause a link-time error.
How complex would it be to import BUILD_BUG() in Xen? Would the 
following work:
#define BUILD_BUG() BUILD_BUG_ON(1)

>   - Replace calls to VM_BUG_ON() with BUG_ON() as we don't currently
>     have a VM_BUG_ON() macro in Xen.
>
>   - Pull in the timeout variants of cmpxchg from the original Xen
>     arm64 cmpxchg.h as these are required for guest atomics and are
>     not provided by Linux. Note these are always using LL/SC so we
>     should ideally write LSE versions at some point.
The main advantage of LSE support in Xen is to drop the timeout helpers. 
I guess this would require a bit more thought to still allow inlining.
In any case, it may make sense to move them in a separate headers so you 
don't have to remove/add them. Anyway, I would be happy with the current 
approach as well.
Cheers,

--
Julien Grall