Re: [Xen-devel] [RFC PATCH v1] Consider void entries in the P2M as 1-1 mapping.

[Ian Campbell <Ian.Campbell@citrix.com>]

On Tue, 2010-12-21 at 21:37 +0000, Konrad Rzeszutek Wilk wrote:
> In the past we used to think of those regions as "missing" and under
> the ownership of the balloon code. But the balloon code only operates
> on a specific region. This region is in lastE820 RAM page (basically
> any region past nr_pages is considered balloon type page). 

That is true at start of day but once the system is up and running the
balloon driver can make a hole for anything which can be returned by
alloc_page.

The following descriptions seem to consider this correctly but I just
wanted to clarify.

I don't think it's necessarily the last E820 RAM page either, that's
just what the tools today happen to build. In principal the tools could
push down a holey e820 (e.g. with PCI holes prepunched etc) and boot the
domain ballooned down such that the N-2, N-3 e820 RAM regions are above
nr_pages too.

> This patchset considers the void entries as "identity" and for balloon
> pages you have to set the PFNs to be "missing". This means that the
> void entries are now considered 1-1, so for PFNs which exist in large
> gaps of the P2M space will return the same PFN.

I would naively have expected that a missing entry indicated an
invalid/missing entry rather than an identity region, it just seems like
the safer default since we are (maybe) more likely to catch an
INVALID_P2M_ENTRY before handing it to the hypervisor and getting
ourselves shot.

In that case the identity regions would need to be explicitly
registered, is that harder to do?

I guess we could register any hole or explicit non-RAM region in the
e820 as identity but do we sometimes see I/O memory above the top of the
e820 or is there some other problem I'm not thinking of?

> The xen/mmu.c code where it deals with _PAGE_IOMAP can be removed, but
> to guard against regressions or bugs lets take it one patchset at a
> time.

Could we have a WARN_ON(_PAGE_IOMAP && !PAGE_IDENTITY) (or whatever the
predicates really are) in some relevant places in mmu.c?

Ian.