Re: [Xen-devel] [PATCH 2/2] xen/mm: Introduce {G, M}FN_INVALID_INITIALIZER

[Tim Deegan <tim@xxxxxxx>]

At 10:33 +0100 on 27 Jun (1498559600), Julien Grall wrote:
> The current implementation of {G,M}FN_INVALID cannot be used to
> initialize global variable because the initializer element is not a
> constant.
> 
> Due to a bug in GCC 4.9 and older ([1]), it is not easy to find a common
> value to initialize a variable and directly passed as an argument.
> 
> Introduce 2 news define {G,M}FN_INVALID_INITIALIZER to be used for
> initializing a variable.
> 
> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64856
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

Acked-by: Tim Deegan <tim@xxxxxxx> (and Ack for the revert too)
but please choose either { ~0UL } or {~0UL} and use it for both.

> ---
>     Build tested it with:
>         * ARM: GCC 4.9.4, 5.1, 4.3, 6.1.1, 7.1.0
>         * x86: Clang 3.5.0, 3.6.0, 3.6.2, 3.8.0, 3.9.0, 4.0.0
> 
>     With introducing a dummy global variable common/mm.c:
> 
>     mfn_t foo = INVALID_MFN_INITIALIZER
> ---
>  xen/include/xen/mm.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/xen/include/xen/mm.h b/xen/include/xen/mm.h
> index 0050fba498..251db4ffa1 100644
> --- a/xen/include/xen/mm.h
> +++ b/xen/include/xen/mm.h
> @@ -57,6 +57,11 @@
>  TYPE_SAFE(unsigned long, mfn);
>  #define PRI_mfn          "05lx"
>  #define INVALID_MFN      _mfn(~0UL)
> +/*
> + * To be used for global variable initialization. This workaround a bug
> + * in GCC < 5.0.
> + */
> +#define INVALID_MFN_INITIALIZER {~0UL}
>  
>  #ifndef mfn_t
>  #define mfn_t /* Grep fodder: mfn_t, _mfn() and mfn_x() are defined above */
> @@ -90,6 +95,11 @@ static inline bool_t mfn_eq(mfn_t x, mfn_t y)
>  TYPE_SAFE(unsigned long, gfn);
>  #define PRI_gfn          "05lx"
>  #define INVALID_GFN      _gfn(~0UL)
> +/*
> + * To be used for global variable initialization. This workaround a bug
> + * in GCC < 5.0 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64856
> + */
> +#define INVALID_GFN_INITIALIZER { ~0UL }
>  
>  #ifndef gfn_t
>  #define gfn_t /* Grep fodder: gfn_t, _gfn() and gfn_x() are defined above */
> -- 
> 2.11.0
> 

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