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

["Durrant, Paul" <pdurrant@xxxxxxxxxxxx>]

> -----Original Message-----
> From: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> Sent: 06 March 2020 11:53
> To: Jan Beulich <jbeulich@xxxxxxxx>; Durrant, Paul <pdurrant@xxxxxxxxxxxx>
> Cc: julien@xxxxxxx; andrew.cooper3@xxxxxxxxxx; sstabellini@xxxxxxxxxx; 
> konrad.wilk@xxxxxxxxxx;
> Volodymyr_Babchuk@xxxxxxxx; ian.jackson@xxxxxxxxxxxxx; wl@xxxxxxx; 
> george.dunlap@xxxxxxxxxx; xen-
> devel@xxxxxxxxxxxxxxxxxxxx
> Subject: RE: [EXTERNAL][PATCH 2/2] domain: use PGC_extra domheap page for 
> shared_info
> 
> 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.
> 

... so getting rid of shared xenheap pages altogether just makes life easier.

  Paul

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