Oh, and I guess I misinterpreted what -mpx was for. To be clear, I was
running into 2 issues:
1. Hyper-V fails to initialize.
"Fixed" by one of:
a) using named cpu model
b) cpu=host and running `bcdedit /set xsavedisable 1` in Windows before
enabling Hyper-V
c) cpu=host,-mpx
d) my hack-y patch from earlier
(b) just tells Hyper-V to disable XSAVE support for its (nested)
guests altogether whereas (c) is more fine=grained and just disables the
BNDCFGx bits.
2. Hyper-V initializes but Windows bootloops. I only seem to run into
this with 5.8 but not 5.4 or 5.10.
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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
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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