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Re: [Xen-devel] yanked share, round 2

You avoid zombies and either side can break the sharing without causing harm to the other side. Domains restarting are transparent to either end (the restarting domain just reattachs and keeps going). It avoids the general ownership problem altogether.


Anthony Liguori

King, Steven R wrote:

Hi Anthony -- Can you explain why this is ideal?  I prefer that sharers
and mappers have their own skin the game--that way, Xen doesn't have to
manage a pool and nobody has to worry about the pool being depleted.

-----Original Message-----
From: Anthony Liguori [mailto:aliguori@xxxxxxxxxx] Sent: Friday, January 13, 2006 11:23 AM
To: King, Steven R
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] yanked share, round 2

An ideal solution to this problem would be to keep a separate pool of
shared memory that neither domain owned.  That removes any concerns
about ownership.


Anthony Liguori

King, Steven R wrote:

Hi folks,
A previous thread discussed complications around DomU's sharing memory

pages with each other:

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?


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