Re: CPU hot-plug and RAM hot-add in virtual machines

From: Johannes Totz <>
Date: Wed, 21 Jul 2021 20:56:58 UTC
On 21/07/2021 13:07, Miroslav Lachman wrote:
> On 21/07/2021 12:45, Andriy Gapon wrote:
>> On 21/07/2021 11:53, Miroslav Lachman wrote:
>>> On 09/07/2021 13:16, Andriy Gapon wrote:
> [..]
>>> Thank you for your reply. I know nothing about system internals. Is 
>>> it too much work to implement it or just nobody from developers need 
>>> it thus nobody write the code? Virtualization is more and more 
>>> popular these days so I think it will be useful for many users.
>> In my opinion, both would be big changes.
>> At the moment we do not have any support for offlining / onlining CPUs 
>> and that would involve a lot of careful changes like safely updating 
>> CPU sets, various CPU masks, etc.
>> Ditto for the memory.  Updating page arrays, free memory accounting 
>> structures, possibly some memory maps, etc.
>> Definitely not a weekend project.
> Thank you again. It really seems too complicated.

Have you tried messing around with virtio_balloon(4)?
It's the other way around though. You'd overprovision the VM, then 
balloon memory away and later on give it back, if needed.