Re: Copy-on-write memory to allow many more xenU domains per machine

[Jacob Gorm Hansen <jacob@melon.dk>]

  Michael Vrable wrote:

 > My current status: I've been working first on using shadow page tables
 > to hide the actual machine addresses of pages from domains.  I'm only
 > partway through doing that, and may yet go back and change how I'm
 > approaching that (I'm still learning bits about how Xen works as I'm
 > going through to modify the code; I'm also thinking now of changes that
 > would be good to make copy-on-write easier later).  I'm trying to write
 > up some of my design thoughts; if anyone here is interested or would
 > like to offer comments, I can post that here.
 >
 > David mentioned implementing a clone() for VMs; that is something that
 > I'd also like to try out.  (It should be possible to test that after
 > only the first step of hiding machine addresses, before copy-on-write is
 > implemented, though copy-on-write will make it efficient.

What about having a special variant of the pte-update operation, which 
takes as input a pointer to a pte-entry, and will change that entry into 
a read-only mapping of the same page contents, though perhaps with a 
different frame number in the pte.

Then Xen can lookup the page contents in a global hashtable or binary 
tree (which might be faster, because you don't need to scan the entire 
page contents unless you have a match), and see if there is a matching 
copy somewhere. If there is, it can decrease a refcount for the page 
provided by the user, change the mapping to point to the matching page, 
and free the user's page if the refcount has reached zero.

In this way, domains become responsible for sharing memory across the 
system, but only pages that are likely candidates for sharing (kernel 
text, user process text) are actually scanned, and you don't get the 
overhead of shadow page tables.

Jacob


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