Re: [Xen-devel] [PATCH 2/2] domain: use PGC_extra domheap page for shared_info

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Fri, 2020-03-06 at 12:37 +0100, Jan Beulich wrote:
> I've started looking at the latest version of Paul's series, but I'm
> still struggling to see the picture: There's no true distinction
> between Xen heap and domain heap on x86-64 (except on very large
> systems). Therefore it is unclear to me what "those pages" is actually
> referring to above. Surely new Xen can't be given any pages in use
> _in any way_ by old Xen, no matter whether it's ones assigned to
> domains, or ones used internally to (old) Xen.

Old and new Xen do not coexist. There is a kexec (via kexec_reloc.S and
purgatory) from old to new.

There are some pages which new Xen MUST NOT scribble on, because they
actually belong to the domains being preserved. That includes the EPT
(or at least IOMMU) page tables.

I suppose new Xen also mustn't scribble on the pages in which old Xen
has placed the migration information for those domains either. At
least, not until it's consumed the data.

Anything else, however, is fine for new Xen to scribble on. Fairly much
anything that the old Xen had allocated from its xenheap (and not
subsequently shared to a guest, qv) is no longer required and can be
treated as free memory by the new Xen, which now owns the machine.

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