Re: [Xen-devel] [PATCH 3/4] xen: introduce __symbol

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

Please use scripts/get_maintainers.pl to CC relevant maintainers.

On 15/10/2018 10:56, Stefano Stabellini wrote:
> Introduce a macro, __symbol, which is a simple wrapper around RELOC_HIDE
> to be used everywhere symbols such as _stext and _etext are used in the
> code.
>
> RELOC_HIDE is needed when accessing symbols such as _stext and _etext
> because the C standard forbids comparisons between pointers pointing to
> different objects. _stext, _etext, etc. are all pointers to different
> objects from ANCI C point of view.
>
> To work around potential C compiler issues (which have actually
> been found, see the comment on top of RELOC_HIDE in Linux), and to help
> with certifications, let's introduce some syntactic sugar to be used in
> following patches.
>
> Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
> ---
>   xen/include/asm-arm/mm.h   | 6 ++++++
>   xen/include/asm-x86/page.h | 6 ++++++
>   2 files changed, 12 insertions(+)
>
> diff --git a/xen/include/asm-arm/mm.h b/xen/include/asm-arm/mm.h
> index 940b74b..02ce05a 100644
> --- a/xen/include/asm-arm/mm.h
> +++ b/xen/include/asm-arm/mm.h
> @@ -284,6 +284,12 @@ static inline uint64_t gvirt_to_maddr(vaddr_t va, paddr_t 
> *pa,
>   #define __mfn_to_virt(mfn) (maddr_to_virt((paddr_t)(mfn) << PAGE_SHIFT))
>   
>   /*
> + * Use RELOC_HIDE with symbols such as _stext and _etext to avoid errors
> + * on comparing pointers to different objects
> + */
> +#define __symbol(x)         ((char *)RELOC_HIDE((unsigned long)(x), 0))
There are no different between arm and x86. A better place would be 
xen/compiler.h (or something common).
But, after this patch, there are even more chance the compiler will 
consider that 2 _symbol(...) will come from different objects. So how is 
this meant to help here?
Cheers,

--
Julien Grall

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