[Xen-devel] yanked share, round 2

["King, Steven R" <steven.r.king@xxxxxxxxx>]

Hi folks,

A previous thread discussed complications around DomU's sharing memory pages with each other:

http://lists.xensource.com/archives/html/xen-devel/2005-12/msg00499.html

 

To summarize, DomU's get into trouble, e.g. unable to shutdown, unless the remote DomU's play nice.  Since DomU's do not trust each other, that is problematic.  I'd like to discuss how to clean away this dependency.

 

Here's one idea.  The goal is to robustly decouple the sharing and remote domains.

 

Grant tables add a new GTF_safe flag, settable by the sharing DomU.

In order to map a GTF_safe page, a remote domain must provide a page of its own, which I'll call an "under page".

Xen holds the under-page on behalf of the remote DomU and maps the shared page into the remote DomU's machine.

At any time, the sharing DomU can unshare the page, crash, etc, which ends ALL foreign access to that page, not just new mappings.

For each remote domain that still maps the unshared page, Xen maps the remote's under-page in place of the unshared page.

The remote domain can unmap at any time and recover its under-page.

 

The purpose of the under-page is to plug the memory hole in the remote DomU created by a surprise unsharing.  A nervous remote DomU could check that a share is GTF_safe before proceeding to map the page.

 

Good, bad or ugly?

-steve

 

 

 

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