Hi there again,
just wanted to ask, why I'm getting no answers? Everyone's busy?
Has this already been discussed (I wouldn't actually believe, if I see
how Qemu behaves for example in scenario 1)?
If you'd like to have more details about the issue or anything else,
please contact me!
Thanks!
Moritz Duge
05.08.2010 13:41 by Moritz Duge:
Hi,
I had some trouble while using the ballooning feature of KVM (using
Ubuntu 10.04 with standard software versions).
The first scenario:
1. Having a guest started by this command: qemu -enable-kvm -m 768
-balloon virtio -cdrom linux_2.6.34.iso
The guest is running Linux 2.6.34 including the ballooning driver.
2. Entering "balloon 256" and a few seconds later "info balloon" in
the Qemu monitor. Qemu will report, the guest uses 256mb of memory
now. The guest is reporting the same (using "free" for example).
3. Entering "change ide1-cd0 linux_2.6.18.iso" to change the guests
CD-ROM to another image, containing a Linux kernel without ballooning
driver.
4. Rebooting the guest.
5. After booting the 2.6.18-OS, it will report it has 768mb memory
(using "free). But Qemu monitor will still tell 256, when entering
"info memory".
I know why this happens. But is this a good behaviour? Shouldn't Qemu
tell something like "maybe 256, but there is no more balloon driver in
the guest and maybe it uses the full 768 now"???
The second scenario:
After the first scenario, the guest can also really start using the
additional 512mb of memory (768 - 256)!!! I think this shouldn't
happen or at least there should be an option to allow or deny this. Or
at least least least this should be printed in big letters in the
man-pages or somewhere else where everyone will read it!
Because before I experienced this, I assumed I can be sure the guest
can't get back the memory which was freed using ballooning. So if I
use the memory freed by ballooning for some other qemu-instances and
the first one starts to use those memory again, all Qemu instances
will crash (this is what actually happens in most cases).
What I'm asking for, is a way to force the guest to stay in the memory
I assigned by ballooning. And if the guest tries to use more memory
(maybe because it just unloaded the ballooning driver) the guest
should crash, but the host shouldn't get in any trouble!!!
This can be really annoying. I think a very common use-case for
virtualization is, to run untrusted software or unsecure webservices
in a vm, so the bad software can't do anything to the host or other
VMs on the host. But when using ballooning, the bad software can! It's
no "remote code execution", but the guest can consume a lot of memory
and cause the host or at least the other VMs on the host to crash.
The third scenario:
1. Booting a machine with a guest not having a ballooning driver.
(e.g. qemu -enable-kvm -m 768 -balloon virtio -cdrom linux_2.6.18.iso)
2. Adjusting the memory by "balloon 512" in the qemu monitor.
3. Qemu won't report that it couldn't adjust the memory. Instead it
will wait until the guest loads a ballooning driver. Is this a good
behaviour? Shouldn't there be at least a switch in the qemu monitor
for the command "balloon". So if I use the switch when changing the
memory (e.g. "balloon -h 256"), qemu won't try to change the memory
later and it will tell me "error: no ballooning driver found".
Thanks for reading and thanks a lot, if there will be a solution for
this, specially for scenario two.
Greetings
Moritz Duge
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