Re: [Xen-devel] [PATCH 7/7] x86: add iommu_ops to map and unmap pages, and also to flush the IOTLB

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
[snip]
> >> How are you making sure this is a mapping that was established via
> >> the map op? Without that this can be (ab)used to ...
> >>
> >> > +    put_page(page);
> >>
> >> ... underflow the refcount of a page.
> >>
> >
> > Yes, I guess I need to ensure that only non-RAM (i.e. RMRR and E820
> reserved
> > areas) are mapped through the IOMMU or this could indeed be abused.
> 
> Now I'm confused - then you don't need to deal with struct page_info
> and page references at all. Nor would you need to call
> get_page_from_gfn() and check p2m_is_any_ram(). Also - what use
> would the interface be if you couldn't map any RAM?
> 

Sorry to confuse...

What I meant was that safety (against underflow) is predicated on 
iommu_lookup_page() failing if the mapping was not established through an iommu 
op hypercall. So, the only things that should be valid in the iommu (and hence 
that iommu_lookup_page() would succeed for) at the point where the guest starts 
to boot must all fall within reserved regions, so thay they are ruled out by 
the earlier check.

  Paul
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