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[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

Nested kvm on Intel (vmx) unfortunately saw quite a bit of regression starting with kernel v3.10 by
  commit 5f3d5799974b89100268ba813cec8db7bd0693fb
  KVM: nVMX: Rework event injection and recovery
Then there were several changes to nested VMX until v3.12 where things seemed to work again. Sounds a bit like 3.13 again does something bad. Saucy problems would be bug #1208455 and there is another issue right now with 32bit kvm on Trusty hosts which is tracked as bug #1268906 (just for having references).

We need to see what we can do about Saucy, the problem is that v3.11
sits right in the middle of meddling around with nested VMX. So going
back may require as much change as going forward. And either way is a
risk (for other regressions).

The message about zapping shadow pages looks to be rather some forgotten
debug code. Some index is initialized in a way that causes that to
happen quite early and is supposed to ensure that case is tested (maybe
it still is not, who knows, but should be less likely).

>From your description it sounds like some nested VMX (again) but just to make sure I got this right. The failing combination is:
- Host: P user-space, T kernel;  Lvl1: P user-space, P kernel;  Lvl2: T user-space, T kernel
- Host: T user-space, T kernel;  Lvl1: T user-space, T kernel;  Lvl2: T user-space, T kernel
Is that correct or did I get that wrong?

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https://bugs.launchpad.net/bugs/1278531

Title:
  nested kvm fails with trust and upstream kernels

Status in “linux” package in Ubuntu:
  Confirmed

Bug description:
  First: the 3.2 precise kernel handled nested qemu very well.  As of
  saucy it has declined.

  In a host with saucy kernel (even on precise userspace), attempts to
  do nested kvm result in a hung kvm (inside the guest - host proceeds
  ok) taking 100%cpu.

  In a host with trusty kernel (even on precise userspace), nested kvm
  fails get past grub.  I have two screenshots, one resulting from
  attempting to boot from a precise mini-iso, another from attempting to
  boot a cloud image at:  http://cloud-images.ubuntu.com/quantal/current
  /quantal-server-cloudimg-amd64-disk1.img.  (If you convert that image
  to raw, it fails the same way).

  On the host, I see the following in /var/log/kern.log: kvm: zapping
  shadow pages for mmio generation wraparound

  I've reproduced this both on (a) a intel based vostro laptop - with
  separate installs of precise and saucy (with ubuntu precise, saucy,
  trust, upstream kernels), (b) an intel based server with precise
  userspace and saucy and trusty kernels;  and (c) an intel laptop
  running full uptodate trusty.

  As nested qemu worked will in the previous LTS, I think it is
  important to have it working in 14.04 LTS.

  ============= Original description ================
  I have a precise host with saucy ubuntu kernel installed.  I installed two VMs there, a saucy and a trusty guest.

  In the saucy guest, non-accelerated qemu works fine, but accelerated
  kvm hangs the first-level saucy guest completely, and pins it at 200%
  cpu usage:

     qemu-system-x86 --enable-kvm -monitor stdio -vnc :1

  On the trusty guest it works just fine.

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