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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.05.17 at 17:33, <jgross@xxxxxxxx> wrote:
> 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?

Oh, indeed. Yet still, guests not intended to be migrated could be
built that way. I don't think we should allow kernel side relaxation
here.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel