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

[Juergen Gross <jgross@xxxxxxxx>]

On 20/09/17 16:24, Julien Grall wrote:
> 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@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?
> 
> 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.

Is there any memory hotplug possible (host level, guest level)?

> 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.

Okay, so this is similar to x86 HVM then.

> 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.

I guess this information is included in some tables like ACPI or DT?


Juergen


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