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Re: [Xen-devel] [PATCH v3 5/6] mm: add 'is_special_page' macro...

> -----Original Message-----
> From: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
> Sent: 05 March 2020 15:10
> To: pdurrant@xxxxxxxx
> Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>; Durrant, Paul 
> <pdurrant@xxxxxxxxxxxx>; Jan Beulich
> <jbeulich@xxxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; Wei Liu 
> <wl@xxxxxxx>; Roger Pau Monné
> <roger.pau@xxxxxxxxxx>; George Dunlap <george.dunlap@xxxxxxxxxx>; Ian Jackson
> <ian.jackson@xxxxxxxxxxxxx>; Julien Grall <julien@xxxxxxx>; Konrad Rzeszutek 
> Wilk
> <konrad.wilk@xxxxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>; Tim 
> Deegan <tim@xxxxxxx>
> Subject: RE: [EXTERNAL][PATCH v3 5/6] mm: add 'is_special_page' macro...
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> On Thu, Mar 5, 2020 at 5:45 AM <pdurrant@xxxxxxxx> wrote:
> >
> > From: Paul Durrant <pdurrant@xxxxxxxxxx>
> >
> > ... to cover xenheap and PGC_extra pages.
> >
> > PGC_extra pages are intended to hold data structures that are associated
> > with a domain and my be mapped by that domain. They should not be treated
> > as 'normal' guest pages (i.e. RAM or page tables). Hence, in many cases
> > where code currently tests is_xen_heap_page() it should also check for
> > the PGC_extra bit in 'count_info'.
> >
> > This patch therefore defines is_special_page() to cover both cases and
> > converts tests if is_xen_heap_page() to is_special_page() where
> > appropriate.
> In context of VM forking, are these pages only used by some type of PV
> mechanism? If not, would we need to get them copied somehow or are
> these setup during the regular createdomain routine? Can they be
> copied on-demand, ie. do these pages pass a p2m_is_ram() check?

PGC_extra domheap pages are intended as direct replacements for shared xenheap 
pages and should be treated the same way. Thus they do not form part of the 
migration stream. Their p2m type depends entirely on how they are added to the 
p2m, as it is for any other page.


> Thanks,
> Tamas
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