Re: memory hotplug for guests?

[Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>]

On Thursday 05 April 2007, Dor Laor wrote:
> >> >For example, lets says you are running several guests, and would
> like
> >> to
> >> >start yet another one for a while - but have no free memory left.
> >> >
> >>
> >> We have another solution for it that will soon be pushed into the
> >> kernel:
> >> It is the balloon driver solution.
> >> Each guest runs a balloon driver, when the host needs to free up
> memory
> >> a daemon with certain policy asks some of the guests to inflate their
> >> balloon,
> >> KVM frees their ballooned pages and the host free memory increases.
> >> When the memory pressure relives, the balloons get deflate command.
> >
> >You probably want to have a look at arch/s390/mm/cmm.c which is
> >exactly doing what you want. Of course the message interface to the
> >hypervisor is different to what you want to do.
> 
> Gee, thanks, I wasn't aware of it. I wrote the Linux driver, not yet
> ready for mainline. This might change things and I'll might use/change
> the cmm.
> It will take a while because my queue is jammed with too much stuff.

Another driver you might want to look at is the Xen balloon driver:
http://81.161.245.2/lxr/http/source/linux-2.6-xen-sparse/drivers/xen/balloon/balloon.c

The s390 driver is probably a better example though. One thing the
Xen one got wrong is that its interface requires you do an hcall in
order to get back a page that you gave up to the host. On s390, it
simply maps the host empty_zero_page as COW into the guest like
the initial memory, so you get a new page back with the regular
method whenever you write to the guest real page.

	Arnd <><

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