Re: [Xen-devel] [PATCH for-4.5] xen: vnuma: expose vnode_to_pnode to guest

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Nov 10, 2014 at 12:42:07PM +0100, Dario Faggioli wrote:
> On Mon, 2014-11-10 at 11:09 +0000, Wei Liu wrote:
> > On Mon, Nov 10, 2014 at 11:51:28AM +0100, Dario Faggioli wrote:
> >
> > > I'm 100% ok to re-start that discussion here and now... however, how
> > > stable should this interface be? Can't we deal with this when actually
> > > implementing NUMA aware ballooning and add stuff at than point, if
> > > necessary?
> > > 
> > The risk would be any new guests with extended get_vnumainfo interface
> > won't be able to run on old hypervisor (4.5) without proper versioning.
> > 
> Right.
> 
> > So basically we have three choices:
> > 1. Expose vnode_to_pnode in hypercall interface.
> > 2. Expose the mapping in xenstore.
> > 3. Don't expose anything, everything happens automagically without guest
> >    knowing anything.
> > 
> > I'm fine with any of those three. However, Jan suggested in that one
> > year old thread it would be wrong for the guest to know the mapping, so
> > I think he implicitly voted for the third option.
> > 
> Option 3 is the best IMO too. The guest, ideally, should know nothing
> about how Xen mapped its virtual NUMA nodes onto the host RAM.
> 

Agreed. I would go for this approach personally.

> The question here is how effective that is. As of now, it's quite hard
> to judge whether we'll be able to do everything automatically, I think.
> I read your proposal, and it looks sensible, I'm just saying it's hard
> to be conclusive at this stage.
> 

I just checked the code. Unfortunately increase_reservation cannot cope
with my proposal.

Currently, the increase_reservation hypercall only allocates pages with
no prior knowledge of what part of guest physical address space those
pages will be added to.

So for NUMA-aware ballooning, we might need a increase_reservation2
hypercall that does the trick I mentioned.

Wei.

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