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Re: [Xen-devel] [PATCH] Device duplicate check fix

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] [PATCH] Device duplicate check fix
From: Michal Novotny <minovotn@xxxxxxxxxx>
Date: Wed, 03 Jun 2009 10:45:08 +0200
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Hi,
you don't see the need to add this check to xend since the same check is already present in the hotplug scripts? So, is this useless and should I do nothing about that and let it be reverted like Keir wrote he did and do nothing about that?

Michal

Stefano Stabellini wrote:
Sorry for the late reply, but I only now I realized that this patch
breaks stubdoms: an exception is needed to allow a disk to be shared
between the guest and its own stubdom.

Beside I do not see the need to add this check to xend since the same
check is already present in the hotplug scripts.

Michal Novotny wrote:

*Hello,
I've checked the duplicate-check code here and I found that's checked only in the context of one domain but not cross-domain. The thing is that we should check tap/vbd device cross-domain not to allow another guest to use the same disk image in some circumstances to prevent VM's disk corruption.

The patch included denies disk image addition under those circumstances:
 1. **We're adding read-only disk that's already used as write-exclusive
 2. **We're adding write-shared disk that's already used as write-exclusive
 3. **We're adding write-exclusive disk that's already used
4. **We're adding read-only disk that's already used as write-shared* (because of I/O caching issues etc.)
*
The vif device duplicate check remains the same it was and it's checked in the context of current domain only so that behaviour has been preserved.

Michal

Signed-off-by: Michal Novotny <minovotn@xxxxxxxxxx>
*





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