Re: [Xen-devel] [V9 PATCH 6/8] pvh dom0: Add and remove foreign pages

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

On Tue, 22 Apr 2014 08:28:45 +0100
"Jan Beulich" <JBeulich@xxxxxxxx> wrote:

> >>> On 22.04.14 at 02:19, <mukesh.rathor@xxxxxxxxxx> wrote:
> > On Wed, 16 Apr 2014 18:37:42 -0700
> > Mukesh Rathor <mukesh.rathor@xxxxxxxxxx> wrote:
> > 
> >> On Wed, 16 Apr 2014 17:00:35 +0100
> >> "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> >> 
............

> >      /* following will take a refcnt on the mfn */
> >      page = get_page_from_gfn(fdom, fgfn, &p2mt, P2M_ALLOC);
> > -    if ( !page || !p2m_is_valid(p2mt) )
> > +    if ( !page || !p2m_is_strict_ram(p2mt) )
> >      {
> >          if ( page )
> >              put_page(page);
> > diff --git a/xen/include/asm-x86/p2m.h b/xen/include/asm-x86/p2m.h
> > index 47604da..e0bf6dc 100644
> > --- a/xen/include/asm-x86/p2m.h
> > +++ b/xen/include/asm-x86/p2m.h
> > @@ -165,6 +165,7 @@ typedef unsigned int p2m_query_t;
> >  
> >  /* Useful predicates */
> > +#define p2m_is_strict_ram(_t) (p2m_to_mask(_t) &
> > p2m_to_mask(p2m_ram_rw))
> 
> Not sure - this now precludes mapping log-dirty or r/o pages, i.e. I
> think you went from too relaxed to too strict. I think the question
> needs answering in a more abstract fashion first, i.e. without code
> written immediately: Which pages absolutely need to be foreign-
> mappable and which ones can safely get foreign-mapped. My first
> thought would be P2M_RAM_TYPES & ~P2M_HOLE_TYPES, with
> P2M_HOLE_TYPES contained in P2M_RAM_TYPES generating a kind
> of -EAGAIN notification while triggering the re-creation of the page
> (which already is an indication that instead of P2M_HOLE_TYPES it
> might well need to be P2M_PAGING_TYPES).

The current use case is p2m_ram_rw, my approach would be start with 
minimum and add more as needed when features like migration, passthru, etc
are added. My understanding of the various p2m types is limited, so I
am at your mercy here to advise what types to anticipate. IMO, just
checking for P2M_RAM_TYPES might suffice, I can additionally add check
to make sure it's not P2M_HOLE_TYPES. Please lmk.

thanks
mukesh


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