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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.01.15 at 11:41, <david.vrabel@xxxxxxxxxx> wrote:
> On 07/01/15 09:10, Olaf Hering wrote:
>> On Mon, Jan 05, Vitaly Kuznetsov wrote:
>> 
>>> Wei Liu <wei.liu2@xxxxxxxxxx> writes:
>>>
>>>> Olaf mentioned his concern about handling ballooned pages in
>>>> <20141211153029.GA1772@xxxxxxxxx>. 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.

Jan

> 3. Running the kexec'd image in a reserved chunk of memory (the crash
> kernel case).
> 4. Providing balloon information to the kexec'd image.
> 
> None of these require any additional hypervisor or toolstack support and
> 1-3 are trivial for a guest to implement.
> 
> David




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