Re: [PATCH 2/3] docs: document ~/control/feature-balloon

[Yann Dirson <yann.dirson@xxxxxxxx>]

On 5/11/23 11:21, Jan Beulich wrote:
> On 10.05.2023 16:20, Yann Dirson wrote:
>> --- a/docs/misc/xenstore-paths.pandoc
>> +++ b/docs/misc/xenstore-paths.pandoc
>> @@ -509,6 +509,12 @@ This may be initialized to "" by the toolstack and may 
>> then be set
>>   to 0 or 1 by a guest to indicate whether it is capable of responding
>>   to a mode value written to ~/control/laptop-slate-mode.
>>   
>> +#### ~/control/feature-balloon
>> +
>> +This may be initialized to "" by the toolstack and may then be set to
>> +0 or 1 by a guest to indicate whether it is capable of memory
>> +ballooning, and responds to values written to ~/memory/target.
> 
> Besides correctly saying "may", I guess this wants to go further and also
> clarify what the (intended) behavior is when the node is absent. Aiui PV
> guests are always expected to have a balloon driver, so the assumed
> value likely needs to be "1" there. Furthermore I'm afraid it doesn't
> really become clear what value this node is if it's only optionally
> present, while its absence doesn't really allow uniform assumptions
> towards a default value.


Things are indeed more complicated than I originally identified,
the way this xenstore entry is used currently seems to make it difficult 
to introduce it in a backward-compatible manner

I guess this and a number of details ought to be discussed at the XAPI
level first.

Details: the squeezed assumption [1] is that a domain which has not set 
this to 1 is not ready yet to get ballooned, which implies the default 
has to be 0 whatever the guest type, as it requires to know the total 
number of pages used by the domain to be stable.  So I guess we can see 
it as not being "not just a feature flag".


[1] 
https://github.com/xapi-project/xen-api/tree/master/ocaml/squeezed/doc/design#environmental-assumptions


-- 
Yann Dirson | Vates Platform Developer
XCP-ng & Xen Orchestra - Vates solutions
w: vates.tech | xcp-ng.org | xen-orchestra.com


Yann Dirson | Vates Platform Developer

XCP-ng & Xen Orchestra - Vates solutions
w: vates.fr | xcp-ng.org | xen-orchestra.com