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[Xen-devel] Re: Memory-hotplug support for x86_64 domUs?

To: "Radhakrishnan, Prashanth" <Prashanth.Radhakrishnan@xxxxxxxxxx>
Subject: [Xen-devel] Re: Memory-hotplug support for x86_64 domUs?
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Tue, 12 Aug 2008 10:46:30 -0700
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Radhakrishnan, Prashanth wrote:
Thanks for the info!

But it isn't very useful at present.

Can you please elaborate why. Is it not possible to add memory to VMs
without rebooting them?

Sorry, I was unclear. It's definitely possible in principle to add memory to VMs on the fly, but it just isn't implemented in the pvops kernels at the moment. The patch I posted (that you referred to) makes the balloon driver call into the memory hotplug subsystem to add new memory to the system. But the new memory is not populated all at once; it's just a set of unbacked pageframes to start with. The memory hotplug system automatically puts a file in /sys referring to the newly added "memory", and allows you to "online" it all. Unfortunately, since there's no real memory behind the pages, this will almost immediately cause your domain to crash.

I'd like to make it so that the balloon driver can control onlining pages on a page-by-page basis, and there should be no usermode interface for this memory. But so far we haven't worked out mutually acceptible way to do this.

I could post the straightforward balloon/hotplug patches, but I'm not comfortable doing so when they leave such an dangerous pit for people to fall into.


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