Re: [PATCH v4 4/4] xen/mm: Provide dummy M2P-related helpers when !CONFIG_HAVE_M2P

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.09.2020 20:02, Julien Grall wrote:
> --- a/xen/include/xen/mm.h
> +++ b/xen/include/xen/mm.h
> @@ -685,4 +685,17 @@ static inline void put_page_alloc_ref(struct page_info 
> *page)
>      }
>  }
>  
> +/*
> + * Dummy implementation of M2P-related helpers for common code when
> + * the architecture doesn't have an M2P.
> + */
> +#ifndef CONFIG_HAS_M2P
> +
> +#define INVALID_M2P_ENTRY        (~0UL)
> +#define SHARED_M2P(_e)           false
> +
> +static inline void set_gpfn_from_mfn(unsigned long mfn, unsigned long pfn) {}

While I think this would better BUG() or at least ASSERT_UNREACHABLE(),
I realize its use in page_alloc.c prevents this. However, if this was a
macro, I think the need for having INVALID_P2M_ENTRY would vanish, as
long as the stub macro didn't evaluate its 2nd argument.

I'm feeling somewhat uneasy with the SHARED_M2P() definition: This
would seem to better be tied to CONFIG_MEM_SHARING rather than M2P
existence.

Jan