Re: [Xen-devel] Strange (???) xl behavior for save, migrate and migrate-receive

[Daniel Kiper <dkiper@xxxxxxxxxxxx>]

On Tue, Oct 18, 2011 at 09:19:01AM +0100, Ian Campbell wrote:
> On Mon, 2011-10-17 at 19:44 +0100, Dan Magenheimer wrote:
> > In a recent internal discussion at Oracle, we were thinking about
> > whether to enable hotplug functionality in a guest kernel and it
> > raised some concerns about manageability.  I think right now
> > the system administrator of the guest can arbitrarily increase
> > memory size beyond maxmem...
>
> The memory limit for a guest is ultimately controlled by the host
> administrator/toolstack. The in-guest admin cannot exceed that, even
> using hotplug.

Correct.

> I think that limit is currently always set to the current balloon target.

Nope. It is set by maxmem option.

> AIUI Daniel's work only allows a guest admin to take advantage of new
> memory above maxmem _after_ the host admin has provisioned that RAM to
> the guest. IOW it only allows the guest to take advantage of new memory
> given to it and does not allow the guest to acquire new memory of its
> own accord.

Guest/host administartor could allocate for given guest no more memory than
maxmem (its value could be changed by xl mem-max <domain> <new_size>) allows,
regardless of mechanism (ballooning or memory hotplug) used for that allocation.
It means that memory hotplug does not pose any security threat in that area.

Daniel

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