Re: [PATCH 03/10] xen/arm: introduce PGC_reserved

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 20/05/2021 10:27, Jan Beulich wrote:
> On 20.05.2021 10:59, Julien Grall wrote:
> > On 20/05/2021 09:40, Penny Zheng wrote:
> > > Oh, Second thought on this.
> > > And I think you are referring to balloon in/out here, hmm, also, like
> > Yes I am referring to balloon in/out.
> >
> > > I replied there:
> > > "For issues on ballooning out or in, it is not supported here.
> > So long you are not using the solution in prod then you are fine (see
> > below)... But then we should make clear this feature is a tech preview.
> >
> > > Domain on Static Allocation and 1:1 direct-map are all based on
> > > dom0-less right now, so no PV, grant table, event channel, etc, considered.
> > >
> > > Right now, it only supports device got passthrough into the guest."
> > So we are not creating the hypervisor node in the DT for dom0less domU.
> > However, the hypercalls are still accessible by a domU if it really
> > wants to use them.
> >
> > Therefore, a guest can easily mess up with your static configuration and
> > predictability.
> >
> > IMHO, this is a must to solve before "static memory" can be used in
> > production.
> I'm having trouble seeing why it can't be addressed right away: 
It can be solved right away. Dom0less domUs don't officially know they 
are running on Xen (they could bruteforce it though), so I think it 
would be fine to merge without it for a tech preview.
> Such
> guests can balloon in only after they've ballooned out some pages,
> and such balloon-in requests would be satisfied from the same static
> memory that is associated by the guest anyway.
This would require some bookeeping to know the page was used previously. 
But nothing very challenging though.
Cheers,

--
Julien Grall