Re: 2MB page PV guest support clarification

[Rolf Neugebauer <rolf.neugebauer@netronome.com>]

Mick Jordan wrote:
> On 02/27/09 17:28, Ian Pratt wrote:
>>> 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.
>>
>>   
> Ok. I need to check this. Certainly I am at some point taking already 
> mapped pages and using them as pagetables. However, I am not getting any 
> errors when adding the PTE. So perhaps the code does the mapping change 
> already.

In mini-os, new_pt_frame() will update the 1:1 mapping to mark a PT page 
RO before hooking it into the page table.

rolf


> 
> Mick
> 
> 
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