Re: Proposed XENMEM_claim_pages hypercall: Analysis of problem and alternate solutions

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Andres Lagar-Cavilla [mailto:andreslc@gridcentric.ca]

> On Jan 2, 2013, at 4:43 PM, Dan Magenheimer <dan.magenheimer@oracle.com> wrote:
> > I reject the omnisicient toolstack model as unimplementable [1]
> > and, without it, I think you either do need a separate allocation/list,
> > with all the issues that entails, or you need the proposed
> > XENMEM_claim_pages hypercall to resolve memory allocation races
> > (i.e. vs domain creation).
> 
> That pretty much ends the discussion. If you ask me below to reason within the constraints your
> rejection places, then that's artificial reasoning. Your rejection seems to stem from philosophical
> reasons, rather than technical limitations.

Well, perhaps my statement is a bit heavy-handed, but I don't see
how it ends the discussion... you simply need to prove my statement
incorrect! ;-)  To me, that would mean pointing out any existing
implementation or even university research that successfully
predicts or externally infers future memory demand for guests.
(That's a good approximation of my definition of an omniscient
toolstack.)

But let's save that for another time or thread.
 
> Look, your hyper call doesn't kill kittens, so that's about as far as I will go in this discussion.

Noted.  I will look at adding kitten-killing functionality
in the next revision. ;-)

> My purpose here was to a) dispel misconceptions about sharing b) see if something better comes out
> from a discussion between all interested mm parties. I'm satisfied insofar a).

At some time I hope to understand paging/sharing more completely
and apologize, Andres, if I have cast aspersions about its/your
implementation, I was simply trying to use it as another
example of an in-hypervisor page allocation that is not
directly under the control of the toolstack.  I do understand
and agree that IF the toolstack is capable of intelligently
managing d->max_pages across all domains, then your model
for handling CoW hits will be sufficient.

So... again... peace?
Dan