Re: [Xen-devel] [RESEND] Question about recursive mappings

[Mathieu Ropert <mro@xxxxxxxxxx>]

Ian Pratt wrote:

> > About the possible fix, won't something like trying a
> > get_linear_pagetable() at all level works? Will it cause too 
> > much overhead?
> > Generally speaking, is allowing a R/O mapping of another 
> > table the domain own (whichever level it is) safe? At first 
> > thought, i don't see any way of exploiting it.
> >    
> Yes, it's safe, you just need to get the 'general' ref count right,
> which as I recall, depends on whether the page you're mapping is in the
> same page table, or a foreign page table. The va back pointer means that
> there is a unique 'normal' place in each pagetable where a given page
> can be mapped, so you can easily inspect (via xen's linear mapping) to
> see whether the page belongs to the current pagetable or not.
>
> One thing we do have to watch out for is when we introduce super page
> mappings, as you have to be careful about using linear page tables in
> this context -- the x86 pagetable format doesn't allow you to generate a
> trap if a linear mapping attempts to misuse a superpage. That's not your
> worry right now :-) 
>
> Ian
>  
Ok, i've done a little patch which seems to resolve the issue on my test 
setup. Basically, i've modified get_linear_pagetable(), added a level 
parameter and call it for each level but level 1.
I need to do a little more test before i post the patch. As tomorrow is 
a national day, it should be ready friday.
Mathieu

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