Re: [PATCH v5 0/9] toolstack-based approach to pvhvm guest kexec

[Vitaly Kuznetsov <vkuznets@redhat.com>]

"Jan Beulich" <JBeulich@suse.com> writes:

>>>> On 07.01.15 at 11:41, <david.vrabel@citrix.com> wrote:
>> On 07/01/15 09:10, Olaf Hering wrote:
>>> On Mon, Jan 05, Vitaly Kuznetsov wrote:
>>> 
>>>> Wei Liu <wei.liu2@citrix.com> writes:
>>>>
>>>>> Olaf mentioned his concern about handling ballooned pages in
>>>>> <20141211153029.GA1772@aepfle.de>. Is that point moot now?
>>>>
>>>> Well, the limitation is real and some guest-side handling will be
>>>> required in case we want to support kexec with ballooning. But as David
>>>> validly mentioned "It's the responsibility of the guest to ensure it
>>>> either doesn't kexec when it is ballooned or that the kexec kernel can
>>>> handle this". Not sure if we can (and need to) do anything hypevisor- or
>>>> toolstack-side.
>>> 
>>> One approach would be to mark all pages as some sort of
>>> populate-on-demand first. Then copy the existing assigned pages from
>>> domA to domB and update the page type. The remaining pages are likely
>>> ballooned. Once the guest tries to access them this should give the
>>> hypervisor and/or toolstack a chance to assign a real RAM page to them.
>>> 
>>> I mean, if a host-assisted approach for kexec is implemented then this
>>> approach must also cover ballooning.
>> 
>> It is not possible for the hypervisor or toolstack to do what you want
>> because there may not be enough free memory to repopulate the new domain.
>> 
>> The guest can handle this by:
>> 
>> 1. Not ballooning (this is common in cloud environments).
>> 2. Reducing the balloon prior to kexec.
>
> Which may fail because again there may not be enough memory to
> claim back from the hypervisor.
>

Yes, but it may be better to cancel kexec at this point.

-- 
  Vitaly