Re: [Xen-devel] [PATCH v8 04/15] xen: add function for obtaining highest possible memory address

[Julien Grall <julien.grall@xxxxxxx>]

Hi Juergen,

Sorry for the late comment.

On 20/09/17 07:34, Juergen Gross wrote:
> Add a function for obtaining the highest possible physical memory
> address of the system. This value is influenced by:
>
> - hypervisor configuration (CONFIG_BIGMEM)
> - processor capability (max. addressable physical memory)
> - memory map at boot time
> - memory hotplug capability
>
> The value is especially needed for dom0 to decide sizing of grant frame
> limits of guests and for pv domains for selecting the grant interface
Why limiting to PV domain? Arm domain may also need to switch to another 
interface between v1 only support 32-bit GFN.
> version to use.
>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
[...]

> diff --git a/xen/include/asm-arm/mm.h b/xen/include/asm-arm/mm.h
> index cd6dfb54b9..6aa8cba5e0 100644
> --- a/xen/include/asm-arm/mm.h
> +++ b/xen/include/asm-arm/mm.h
> @@ -376,6 +376,11 @@ static inline void put_page_and_type(struct page_info 
> *page)
>   
>   void clear_and_clean_page(struct page_info *page);
>   
> +static inline unsigned long arch_get_upper_mfn_bound(void)
> +{
> +    return 0;
> +}
I am not sure to understand the Arm implementation given the description 
of the commit message.
The guest layout is completely separate from the host layout. It might 
be possible to have all the memory below 40 bits on the host, but this 
does not preclude the guest to have all memory below 40 bits (the 
hardware might support, for instance, up to 48 bits).
Cheers,

--
Julien Grall

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