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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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