[Bug 114851] New: 2.6.32 L1 vm on 4.4 host with nested KVM enabled cannot launch L2 vm

[Bug 114851] New: 2.6.32 L1 vm on 4.4 host with nested KVM enabled cannot launch L2 vm

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=114851

            Bug ID: 114851
           Summary: 2.6.32 L1 vm on 4.4 host with nested KVM enabled
                    cannot launch L2 vm
           Product: Virtualization
           Version: unspecified
    Kernel Version: 4.4+
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: kvm
          Assignee: virtualization_kvm@kernel-bugs.osdl.org
          Reporter: jmontleo@redhat.com
        Regression: No

With nested virtualization enabled it was possible with 4.2 and 4.3 to create
L1 RHEV hypervisors running on RHEL 6.7 that could then launch L2 vm's. With
4.4 this is no longer possible

The baremetal system is Fedora 23, currently running 4.4.5. On this an L1 RHEV
hypervisor runs RHEL 6.7 with kernel 2.6.32. Any L2 VM launched in RHEV with
this combination gets stuck booting and you soon start to see cpu soft lockups
on the L1.

adding a modprobe.d file on the L1 with options kvm_intel vpid=0 regains the
ability to launch VM's. Doing this on the baremetal host causes the L1 to panic
and reboot when you try to launch an L2 VM.

As an additional piece of information it still seems possible to launch vm's on
L1 RHEL 7 Openstack compute nodes, so it may be the combination of 4.4+ with
~2.6.32.

I ripped these out in 4.4.6 and rebuilt the kernel at which I once again no
longer saw vpid in /proc/cpuinfo on the L1 and am able to boot L2 vms again:
https://git.kernel.org/torvalds/c/99b83ac893b84ed1a62ad6d1f2b6cc32026b9e85
https://git.kernel.org/torvalds/c/089d7b6ec5151ad06a2cd524bc0580d311b641ad
https://git.kernel.org/torvalds/c/5c614b3583e7b6dab0c86356fa36c2bcbb8322a0

4.4.4 and 4.5.0 are also affected.

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[Bug 114851] 2.6.32 L1 vm on 4.4 host with nested KVM enabled cannot launch L2 vm

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=114851

Paolo Bonzini <bonzini@gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |bonzini@gnu.org
           Assignee|virtualization_kvm@kernel-b |bonzini@gnu.org
                   |ugs.osdl.org                |

--- Comment #1 from Paolo Bonzini <bonzini@gnu.org> ---
Yes, it is a bug in 2.6.32 which we obviously should work around.  I've sent a
patch to the KVM mailing list.

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