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

Hi Ian,

On 16/09/14 06:27, Ian Campbell wrote:
On Tue, 2014-09-16 at 10:49 +0200, Tamas K Lengyel wrote:

On Tue, Sep 16, 2014 at 12:35 AM, Ian Campbell
<ian.campbell@xxxxxxxxxx> wrote:
         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

         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
         (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.


Those would all be possible solutions. I used the Radix tree
implementation already in Xen as a matter of convenience and because
it has an acceptable overhead. I certainly don't oppose further
optimizing this code, I'm just not sure if it needs to happen now,
provided feature freeze is rapidly approaching. The main concern
should be impact on non-xenaccess code-paths, which I agree are a
showstopper of any feature like this. If you say it can't be merged
unless the xenaccess code-path is also optimized I'm afraid this
series will be postponed till 4.6 as I won't have the time to test out
which approach puts the least overhead on the system under what
usage-scenarios etc in this timeframe.

WRT merging I'm only concerned about the impact on non-xenaccess uses,
the stuff about the xenaccess-on case was just idle wondering, sorry for
not making that clear.

I didn't write this code, but read it multiple time and ask the similar question to Tamas few version ago.

So, the radix tree is only used when the access type of the page is different than access_rwx. This is the default access when xenaccess is not used.

AFAIU, the only overhead we have is the new fields in the arch_domain structure.


Julien Grall

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