[Qemu-devel] [Bug 1826422] Re: Regression: QEMU 4.0 hangs the host (*bisect included*)

[Alex Williamson <alex.l.williamson@gmail.com>]

The change in QEMU 4.0 is only a change in defaults of the machine type,
it can be entirely reverted in the VM config with kernel_irqchip=on or
<ioapic driver='kvm'/> with libvirt.  Using a machine type prior to the
q35 4.0 machine type would also avoid it.  There are no performance
issues with these configurations that would favor using 3.1 over 4.0.

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Title:
  Regression: QEMU 4.0 hangs the host (*bisect included*)

Status in QEMU:
  New

Bug description:
  The commit b2fc91db84470a78f8e93f5b5f913c17188792c8 seemingly
  introduced a regression on my system.

  When I start QEMU, the guest and the host hang (I need a hard reset to
  get back to a working system), before anything shows on the guest.

  I use QEMU with GPU passthrough (which worked perfectly until the
  commit above). This is the command I use:

  ```
  /path/to/qemu-system-x86_64
    -drive if=pflash,format=raw,readonly,file=/path/to/OVMF_CODE.fd
    -drive if=pflash,format=raw,file=/tmp/OVMF_VARS.fd.tmp
    -enable-kvm
    -machine q35,accel=kvm,mem-merge=off
    -cpu host,kvm=off,hv_vendor_id=vgaptrocks,hv_relaxed,hv_spinlocks=0x1fff,hv_vapic,hv_time
    -smp 4,cores=4,sockets=1,threads=1
    -m 10240
    -vga none
    -rtc base=localtime
    -serial none
    -parallel none
    -usb
    -device usb-tablet
    -device vfio-pci,host=01:00.0,multifunction=on
    -device vfio-pci,host=01:00.1
    -device usb-host,vendorid=<vid>,productid=<pid>
    -device usb-host,vendorid=<vid>,productid=<pid>
    -device usb-host,vendorid=<vid>,productid=<pid>
    -device usb-host,vendorid=<vid>,productid=<pid>
    -device usb-host,vendorid=<vid>,productid=<pid>
    -device usb-host,vendorid=<vid>,productid=<pid>
    -device virtio-scsi-pci,id=scsi
    -drive file=/path/to/guest.img,id=hdd1,format=qcow2,if=none,cache=writeback
    -device scsi-hd,drive=hdd1
    -net nic,model=virtio
    -net user,smb=/path/to/shared
  ```

  If I run QEMU without GPU passthrough, it runs fine.

  Some details about my system:

  - O/S: Mint 19.1 x86-64 (it's based on Ubuntu 18.04)
  - Kernel: 4.15
  - `configure` options: `--target-list=x86_64-softmmu --enable-gtk --enable-spice --audio-drv-list=pa`
  - EDK2 version: 1a734ed85fda71630c795832e6d24ea560caf739 (20/Apr/2019)
  - CPU: i7-6700k
  - Motherboard: ASRock Z170 Gaming-ITX/ac
  - VGA: Gigabyte GTX 960 Mini-ITX

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