Re: [Xen-devel] [PATCH] public/elfnote: document non-alignment of relocated init-P2M

[Juergen Gross <jgross@xxxxxxxx>]

On 09/05/17 17:32, Jan Beulich wrote:
>>>> On 09.05.17 at 17:25, <jgross@xxxxxxxx> wrote:
>> On 09/05/17 17:06, Jan Beulich wrote:
>>>>>> On 09.05.17 at 16:36, <jgross@xxxxxxxx> wrote:
>>>> On 09/05/17 16:23, Jan Beulich wrote:
>>>>> Since PV kernels can't use large pages anywa, when the init-P2M support
>>>>> was added it was decided to keep the implementation simple and not
>>>>> align large pages in PFN space. Document this.
>>>>>
>>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>>
>>>>> --- a/xen/include/public/elfnote.h
>>>>> +++ b/xen/include/public/elfnote.h
>>>>> @@ -173,7 +173,9 @@
>>>>>   * The (non-default) location the initial phys-to-machine map should be
>>>>>   * placed at by the hypervisor (Dom0) or the tools (DomU).
>>>>>   * The kernel must be prepared for this mapping to be established using
>>>>> - * large pages, despite such otherwise not being available to guests.
>>>>> + * large pages, despite such otherwise not being available to guests. 
>>>>> Note
>>>>
>>>> Shouldn't the large page usage be limited to dom0?
>>>
>>> Why? Even if the tools right now don't use large pages here, why
>>> should we preclude them wanting to at some point?
>>
>> Those could be of temporary nature only in order not to break migration.
>> So the guest would be forced to split the big pages up anyway. Why
>> create them in the beginning then?
> 
> As long as the migration stream doesn't represent them as 1Gb or
> 2Mb pages, I don't see how they would get in the way of migration.

How would the receiving side allocate the additional page tables?


Juergen


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