Re: [Xen-devel] [PATCH 2/3] xen/err: Support an optional per-arch slide for the error region

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.11.16 at 11:46, <andrew.cooper3@xxxxxxxxxx> wrote:
> --- a/xen/include/xen/err.h
> +++ b/xen/include/xen/err.h
> @@ -15,19 +15,28 @@
>   */
>  #define MAX_ERRNO    4095
>  
> +/*
> + * Individual architectures may wish to slide the error region away from its
> + * default position at the very top of virtual address space.  (e.g. if Xen
> + * doesn't own the range).
> + */
> +#ifndef ARCH_ERR_PTR_SLIDE
> +#define ARCH_ERR_PTR_SLIDE 0
> +#endif
> +
>  static inline bool IS_ERR_VALUE(unsigned long x)
>  {
> -     return x >= (unsigned long)-MAX_ERRNO;
> +     return (x + ARCH_ERR_PTR_SLIDE) >= (unsigned long)-MAX_ERRNO;
>  }
>  
>  static inline void *__must_check ERR_PTR(long error)
>  {
> -     return (void *)error;
> +     return (void *)((unsigned long)error - ARCH_ERR_PTR_SLIDE);

I don't see the need for the inner cast here and ...

>  }
>  
>  static inline long __must_check PTR_ERR(const void *ptr)
>  {
> -     return (long)ptr;
> +     return (long)((unsigned long)ptr + ARCH_ERR_PTR_SLIDE);

... the outer one here.

And then of course this change is pointless without patch 3, which
I have reservations against (will reply there).

Jan


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