RE: [Xen-devel] 2MB page PV guest support clarification

[Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>]

> You remark about aliasing prompts me to ask a general question about
> that. I am currently mapping physical to virtual 1-1 (because that is
> what minis-os has always done) as well as mapping parts of that to
> other areas in virtual memory. Both of these are RW mappings. Is that
> ok? It perfectly possible for me to unmap the 1-1 mappings or make them
> RO if I have to.

Any page that is part of a pagetable must be mapped RO in every mapping to it. 
Attempting to add a page that has RW mappings to a pagetable will fail (either 
when you make the hypercall to add the PTE, or when you pin a constructed 
pagetable or try switching to it).

Thus, you need to be careful with 1:1 maps to remove pages that may become PT 
pages. It's best to have a PT page allocator that tries to allocate PT's from 
contiguous regions and then recycles them.

Ian

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