From mboxrd@z Thu Jan  1 00:00:00 1970
From: Vitaly Kuznetsov <vkuznets@redhat.com>
Subject: Re: [PATCH v5 0/9] toolstack-based approach to pvhvm
	guest kexec
Date: Wed, 07 Jan 2015 12:59:01 +0100
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To: Jan Beulich <JBeulich@suse.com>
Cc: Olaf Hering <olaf@aepfle.de>, Wei Liu <wei.liu2@citrix.com>, Ian Campbell <ian.campbell@citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, Andrew Cooper <andrew.cooper3@citrix.com>, Julien Grall <julien.grall@linaro.org>, Ian Jackson <ian.jackson@eu.citrix.com>, Andrew Jones <drjones@redhat.com>, TimDeegan <tim@xen.org>, David Vrabel <david.vrabel@citrix.com>, xen-devel@lists.xenproject.org, Keir Fraser <keir@xen.org>
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"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