[Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor

[Thomas Huth]

The QEMU project is currently moving its bug tracking to another system.
For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting the bug state to "Incomplete" now.

If the bug has already been fixed in the latest upstream version of QEMU,
then please close this ticket as "Fix released".

If it is not fixed yet and you think that this bug report here is still
valid, then you have two options:

1) If you already have an account on gitlab.com, please open a new ticket
for this problem in our new tracker here:

    https://gitlab.com/qemu-project/qemu/-/issues

and then close this ticket here on Launchpad (or let it expire auto-
matically after 60 days). Please mention the URL of this bug ticket on
Launchpad in the new ticket on GitLab.

2) If you don't have an account on gitlab.com and don't intend to get
one, but still would like to keep this ticket opened, then please switch
the state back to "New" or "Confirmed" within the next 60 days (other-
wise it will get closed as "Expired"). We will then eventually migrate
the ticket automatically to the new system (but you won't be the reporter
of the bug in the new system and thus you won't get notified on changes
anymore).

Thank you and sorry for the inconvenience.


** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1908489

Title:
  qemu 4.2 bootloops with -cpu host and nested hypervisor

Status in QEMU:
  Incomplete
Status in qemu-kvm package in CentOS:
  Unknown

Bug description:
  I've noticed that after upgrading from Ubuntu 18.04 to 20.04 that
  nested virtualization isn't working anymore.

  I have a simple repro where I create a Windows 10 2004 guest and
  enable Hyper-V in it. This worked fine in 18.04 and specifically qemu
  <4.2 (I specifically tested Qemu 2.11-4.1 which work fine).

  The -cpu arg I'm passing is simply:
      -cpu host,l3-cache=on,hv_relaxed,hv_spinlocks=0x1fff,hv_vapic,hv_time

  Using that Windows won't boot because the nested hypervisor (Hyper-V)
  is unable to be initialize and so it just boot loops. Using the exact
  same qemu command works fine with 4.1 and lower.

  Switching to a named CPU model like Skylake-Client-noTSX-IBRS instead
  of host lets the VM boot but causes some weird behaviour later trying
  to use nested VMs.

  If I had to guess I think it would probably be related to this change
  https://github.com/qemu/qemu/commit/20a78b02d31534ae478779c2f2816c273601e869
  which would line up with 4.2 being the first bad version but unsure.

  For now I just have to keep an older build of QEMU to work around
  this. Let me know if there's anything else needed. I can also try out
  any patches. I already have at least a dozen copies of qemu lying
  around now.

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