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Re: [Xen-users] Very technical question about ballooning

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Subject: Re: [Xen-users] Very technical question about ballooning
From: "J. Roeleveld" <joost@xxxxxxxxxxxx>
Date: Fri, 13 Aug 2010 17:03:33 +0200
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On Thursday 12 August 2010 16:38:26 Moritz Duge wrote:
> Hi there!
> I'm having a quite difficult question about the ballooning feature of Xen.
> The scenario is like this: I'm having a dom0 and some domUs. But I don't
> trust the operating-system inside one of the domUs. Please don't ask me
> why I just don't trust this operating-system! I can give you 1001
> reasons for it. This domU operating-system could be managed by an evil
> administrator or it could just be unsecure, so someone can break into it
> and gain root access.
> Nevertheless, I would like to use ballooning for all of the domUs, also
> the untrusted one. Mainly because the memory requirements of the domUs
> change sometimes, but I don't want to reboot them.
> That's why I want to use ballooning. And the added maxmem-values (not
> the memory values) will be more then the physical memory I have.
> So the question is: Does Xen ensure, that the untrusted guest doesn't
> cheats the ballooning model?
> What will happen, if memory is set to 512 mb for example and maxmem is
> 768 mb. And then, the guest just unloads the ballooning stuff from it's
> operating-system kernel.

As far as I noticed, the domU will only be able to see the memory assigned to 
the domU bu the dom0.
If the domU does not have ballooning support, then it can only see the amount 
of memory assigned to it during boot (or just before the ballooning support 
was removed)

Ballooning support is in the kernel and the kernel is, at least on my system 
using PV, located on the dom0. The domU does not have access to the kernel and 
my domUs don't have kernel-module support.

> - Will the guest be able to "see" (by using the linux-command free in
> the guest for example) it's maxmem (768 mb)?

"free" only shows the actual memory the domU can use. eg. what is assigned by 
the dom0.

> - And what will happend, if the guest tries to use it's full maxmem (768
> mb), not just the 512 mb? Will the guest crash???

I don't think it's possible for a domU to use more memory then is actually 
assigned to the domU by dom0.
"maxmem" is only the limit that can be assigned by the dom0 to the domU.

> - What happends if the guest can use maxmem and the whole system (dom0
> and the real hardware computer) runs out of memory? Will the whole real
> computer crash? Or just the malicious domU? Or all the domUs, but not
> the dom0???

Again, in my experience, it is not possible to overassign resources.
maxmem on my system is 50% of the total memory in the machine for each VM.
I have 8 running.

However, I hardly ever assign the full amount to a domU.

If I try to assign more memory then is available to be assigned, xen will 
simply reduce the amount to be assigned to the max possible.

I have 1GB free to assign.
I want to assign 2GB to a domU that is already using 1.5GB
This will work, and I then have 512MB left to assign.

I then want to assign 2 GB  to another domU that is already using 1GB.
This domU will then end up with 2GB as that is all that was available.

You can also specify a minimum amount of memory for the dom0. This ensures 
that the dom0, by auto-ballooning down, will not end up with less then this 
minimum. This will ensure the dom0 will remain stable.

> Think of that: In the scenario I'm talking about, the bad domU is not
> really under my control. For shure, I wouldn't use more memory then I
> have. But in this case it's not my decision. It's the decision of
> somebody evil who gained the control over the domU (as I said, don't ask
> me why - there are enough exploids and undiscovered security holes out
> there).

Actually, as I described above, it is your decision as long as you have 
control over the dom0 and the "bad admin" does not have access to the dom0.

> At last:
> - Are there differences concerning this, when using the paravirtualized
> mode (linux) and using the hvm mode with paravirtualized hvm drivers???

I never used HVM, but I doubt they would be able to grab memory from the pool 

> - Are there differences between the versions of the or the available
> xen-linux-kernels?

Yes, but not related to the way resources are granted/revoked to/from domUs.

> - It's not so hard to have a Xen Kernel without ballooning. For example
> look at Fedora 9. It brings a Xen-PV Kernel without ballooning!

Yes, but again, ballooning support only means that the domU is able to support 
the adding/removing of memory by the dom0.
If this support doesn't exist in the domU then you can't increase/decrease the 
In this case, maxmem has no effect and only the specified memory is actually 

> At very last: Is there any detailed documentation for this?

Not sure, what I mentioned above is from my personal experience.


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