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. -- You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. 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. To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1908489/+subscriptions
