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Re: [Xen-devel] xenheap pages mapped by dom0

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] xenheap pages mapped by dom0
From: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Date: Fri, 18 Dec 2009 21:59:21 +0000
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
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On Fri, Dec 18, 2009 at 4:40 PM, Keir Fraser <keir.fraser@xxxxxxxxxxxxx> wrote:
> Once you have called share_xen_page_with_guest(), dom0 can map that page at
> any time and it is not safe to free_xenheap_page() without first checking
> for guest mappings.
> Perhaps you can lazily adjust tarce buffers downwards in size? I.e. Have a
> go, remember if you fail, and try again on future buffer-adjustment
> hypercalls?

I don't expect buffer-adjustment to be a common thing, so the "lazy"
option might end up being incredibly lazy. :-)  My plan now is to do
alloc-once only (i.e., keeping the same semantics), and make all the
allocations before sharing the page with dom0.  In the future, I may
add the tear-down and check for reference count, with an "undo" if
there are still outstanding reference counts.  I think the most likely
reason to run into this scenario is if someone accidentally starts
xentrace or xenmon when there's already an instance of one of them
running, in which case the right thing to do is fail the new one and
let the user decide whether to kill the old process or not.


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