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

[Juergen Gross <jgross@xxxxxxxx>]

On 20/09/17 14:51, Julien Grall wrote:
> 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.

Right. And I just used that reasoning for an answer to Jan. :-)

> 
>> 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).

Who is setting up the memory map for the guest then?


Juergen


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