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

[Julien Grall <julien.grall@arm.com>]

Hi Juergen,

On 20/09/17 14:08, Juergen Gross wrote:
> 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@suse.com>
>>
>> [...]
>>
>>> 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?

The memory map is at the moment static and described in 
public/arch-arm.h. The guest is not allowed to assume it and should 
discover it through ACPI/DT.

There are 2 banks of memory for the guest (it depends on the amount of 
memory requested by the user):
	- 3GB @ 1GB
	- 1016GB @ 8GB

But the guest would be free to use the populate memory hypercall to 
allocate memory anywhere in the address space.

For Arm32, the maximum IPA (Intermediate Physical Address aka guest 
physical address on Xen) we currently support is always 40 bits.

For Arm64, this range from 32 bits to 48 bits. New hardware can support 
up to 52 bits.

Cheers,

-- 
Julien Grall

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