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[Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hyperviso
From: |
Luqman |
Subject: |
[Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor |
Date: |
Sun, 20 Dec 2020 10:36:41 -0000 |
Ok, so I narrowed done one possible issue: the BNDCFGS bits in the vm
entry/exit control MSRs are not set but HyperV expects them to be set if
xsave is supported. This quick patch actually lets Hyper-V initialize
and continue booting:
https://gist.github.com/552baa8be026e67bef2d223076b81636
An alternative to that patch is just telling Hyper-V xsave is disabled.
In the guest before enabling Hyper-V: bcdedit /set xsavedisable 1
Unfortunately while this does let the guest Hyper-V initialize, the
nested (root) Windows guest doesn't boot and still gets stuck in a
bootloop.
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https://bugs.launchpad.net/bugs/1908489
Title:
qemu 4.2 bootloops with -cpu host and nested hypervisor
Status in QEMU:
New
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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- [Bug 1908489] [NEW] qemu 4.2 bootloops with -cpu host and nested hypervisor, Luqman, 2020/12/17
- [Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor, Luqman, 2020/12/17
- [Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor, Luqman, 2020/12/17
- [Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor, Luqman, 2020/12/18
- [Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor,
Luqman <=
- [Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor, Luqman, 2020/12/20
- [Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor, Luqman, 2020/12/20
- [Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor, Luqman, 2020/12/20
- [Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor, Paolo Bonzini, 2020/12/20
- [Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor, Luqman, 2020/12/20
- [Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor, Paolo Bonzini, 2020/12/20
- [Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor, Luqman, 2020/12/20
- [Bug 1908489] Re: qemu 4.2 bootloops with -cpu host and nested hypervisor, Amdnative, 2020/12/23