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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 09/05/17 17:02, Jan Beulich wrote:
>>>> 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.

The save side of PV migrate will cleanly abort if it finds any
superpages, precisely because we have no idea whether the destination
can allocate the frames.

~Andrew

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