Re: [PATCHv1] xen/balloon: disable memory hotplug in PV guests

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 03/09/2015 11:31 AM, David Vrabel wrote:
> On 09/03/15 15:25, Boris Ostrovsky wrote:
>> On 03/09/2015 11:13 AM, David Vrabel wrote:
>>> On 09/03/15 15:09, Boris Ostrovsky wrote:
>>>> On 03/09/2015 10:10 AM, David Vrabel wrote:
>>>>> Memory hotplug doesn't work with PV guests because:
>>>>>
>>>>>      a) The p2m cannot be expanded to cover the new sections.
>>>>>
>>>>>      b) add_memory() builds page tables for the new sections which means
>>>>>         the new pages must have valid p2m entries (or a BUG occurs).
>>>> Is this due to recent p2m rework?
>>> (a) is but (b) has always meant memory hotplug on PV was broken.
>> I can see why (b) should probably have prevented it from working but I
>> tried this on 4.4/3.17 and that seemed to work (it indeed does not
>> function well on newer bits).
> I tried it on 3.10 (+ some other bits and pieces) and it was definitely
> broken for reason (b).  I had to refactor the balloon driver to populate
> the sections before calling add_memory() before it would work.  But this
> can't be done now because of (a).

Could something have happened between 3.10 and 3.17 that fixed it (hard 
to believe but nevertheless)? Or this brokenness depends on Xen version 
(again, unlikely)?

In any case, I think this needs a message in the log stating that 
hotplug doesn't work for this guest (once, so that we don't into 
spamming the log like we have recently).

-boris