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

[Luqman]

Ok, after bisect between stable-4.1 and stable-4.2 I did confirm that
https://github.com/qemu/qemu/commit/20a78b02d31534ae478779c2f2816c273601e869
is the first bad commit.

The full qemu command line is:

qemu-system-x86_64 \
    -name guest=test,debug-threads=on \
    -serial none \
    -enable-kvm \
    -nodefaults \
    -no-user-config \
    -M q35,accel=kvm,kernel_irqchip=on,mem-merge=off \
    -m 8192 -mem-prealloc -no-hpet \
    -cpu 
host,kvm=off,l3-cache=on,hv_relaxed,hv_spinlocks=0x1fff,hv_vapic,hv_time \
    -smp 8,sockets=1,cores=4,threads=2 \
    -global kvm-pit.lost_tick_policy=discard \
    -rtc base=localtime \
    -boot order=c \
    -usb \
    -device pcie-root-port,bus=pcie.0,id=root_port1,chassis=0,slot=0 \
    -device 
vfio-pci,host=01:00.0,id=hostdev1,bus=root_port1,addr=0x00,multifunction=on \
    -device vfio-pci,host=01:00.1,id=hostdev2,bus=root_port1,addr=0x00.1 \
    -drive if=pflash,format=raw,readonly,file=OVMF_CODE.fd \
    -drive if=pflash,format=raw,file=OVMF_VARS.fd \
    -drive if=none,id=drivec,file=disk.img,format=qcow2,cache=none,aio=threads \
    -object iothread,id=iothread1 \
    -device virtio-blk-pci,drive=drivec,scsi=off,iothread=iothread1 \
    -monitor unix:/tmp/monitor.sock,server,nowait \
    -device virtio-mouse-pci,id=input0 \
    -device virtio-keyboard-pci,id=input1 \
    -object 
input-linux,id=kbd1,evdev=/dev/input/by-id/xxxxxxx,grab_all=yes,repeat=on \
    -object input-linux,id=mouse1,evdev=/dev/input/by-id/xxxxxx \
    -netdev tap,ifname=vnet,id=net0,script=no,downscript=no \
    -device e1000,netdev=net0

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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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