Re: [Xen-devel] Re: Xen reboots when trying to start new domain

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

Usually the low mappings are only there to bootstrap the OS out of
non-page-table mode. Once you have page tables, most stuff is going to
transition to the high mappings, surely?

If you just remove the low mappings, I wonder how much code would
actually crash. :-)

Anyway, I've checked in code which, rather than rejecting page tables
that are mapped at multiple different VAs, instead marks the page
information that this is the case. When using shadow page tables, and
potentially writable page tables, this might cause a considerable
slowdown. But that isn't the case for FreeBSD right now of course, so
you could leave teh low mappings for now and strip them out later.

 -- Keir

> Not surprisingly FreeBSD expects those low temporary double mappings to
> be there. How do you recommend fixing this without throwing more
> pagetables at it that pmap_init will have to garbage collect?
> 
>                               -Kip
> 
> 
> 
> 
> 
> >
> > We now support direct update of PTEs by guests. To allow this we have
> > to record which entry in a page directory the page table is mapped
> > at. This means we can no longer support having the same page table
> > mapped at differnt PDE indexes.
> >
> >  -- Keir
> >
> > >
> > >
> > > > That isn't allowed any more. :-)
> > >
> > > Ahh - yes I think it does. I wonder how FreeBSD will fair without that
> > > mapping. Does the aliasing make ref-counting too hard?
> > >
> > >
> > >
> > >
> > >
> > >
> > > >
> > > >  -- Keir
> > > >
> > > >
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