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Re: [Xen-devel] [PATCH for-4.5 v6 10/17] xen/arm: p2m type definitions and changes

On Mon, 2014-09-15 at 16:02 +0200, Tamas K Lengyel wrote:
> Define p2m_access_t in ARM and add necessary changes for page table
> construction routines to pass the default access information. Also,
> define the Radix tree that will hold access permission settings as
> the PTE's don't have enough software programmable bits available.

So my main concern here is the overhead for non-xenaccess users. I think
it amounts to a few extra fields in the p2m_domain struct which I can
see here and presumably some NULL vs. non-NULL type checks which I guess
we will get to later. The important thing is that the fast paths for the
common case don't get a lot of extra overhead.

WRT the xenaccess performance did you consider any options other than a
radix tree (which seems quite expensive to me)? e.g. perhaps allocating
(only when needed) as second page for each real T page as a
shadow/extended region? Perhaps pointed to by a filed in the real PT
struct page_info. I'm sure there are other possible ideas too.


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