Re: [Xen-devel] page counts & type counts

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

Hi, 

At 22:18 -0500 on 04 Jan (1231107492), Mike Sun wrote:
> Does this mean that every time a page goes from read-only to writable,
> the shadow PTE mapping that page is destroyed and recreated?  

No - I mentioned this in another thread, but just for the archives: this
means that every time a page goes from read-only to writable, all
shadows of that page must be destroyed (since it's not safe to let the
guest write directly to a shadowed page). 

> >From what I understand, shadow_remove_all_shadows() will remove all
> shadows of the gmfn, which should be a guest PT.  Basically, it'll
> remove all shadows inserted in the shadow_hash table for that gmfn.
> It IS NOT removing shadow PTEs mapping that gmfn.  Is this a correct
> understanding?

Yes.

> I don't understand how get_page_type() would know in
> the code that is there how the given page is actually a guest PT?

It doesn't, but shadow_remove_all_shadows() does.  As Kevin points out,
in later Xens, this test is modified to check both that the page is
shadowed and also that it's not out of sync before calling out (since
being out of sync is now the one exception to the rule that writeable
pages must not be shadowed).

Cheers,

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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