[Bug 1899733] [NEW] Qemu 5.1.0-1 GPU passthrough - MacOS vm starts only if hdmi is attached

[Daniele]

Public bug reported:

Hi,
I have a macOS 10.15.7 vm with GPU passthrough (NVIDIA GTX Titan Black), 
libvirt xml has path to vbios too.
Qemu 5.1.0-1 with libvirt 6.5.0-2 are installed in manjaro architect 20.1.1 
(two kernels tried: 5.4.67 LTS and 5.8.11 stable, no difference).
I have two monitors, one with hdmi and one with vga inputs.
Usually the gpu is connected to one monitor with hdmi, the other one with DVI 
(on gpu)-->vga adapter, so:
1st monitor: hdmi-->hdmi
2nd monitor: DVI-->vga adapter-->vga

With this setup, launching "virsh start Catalina" has no problem.

If I detach the hdmi cable from monitor 1, I cannot start qemu anymore: the 2nd 
monitor turns black, it doesn't seem it has "no-signal", but only a black 
screen with the power led blinking (usually a window on the monitor floats 
around with "no signal" displayed when there is no signal to monitor).
I say "qemu doesn't start" because in /var/log/libvirt/qemu/Catalina.log 
there's no trace.

Detaching hdmi works with qemu 4.2 and libvirt 5.10, so this could be
related to qemu update.

Apologize, I know there aren't much information here, if someone can
guide me to test the issue I would be grateful.

Thanks for your attention

** Affects: qemu
     Importance: Undecided
         Status: New


** Tags: gpu hdmi passthrough

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https://bugs.launchpad.net/bugs/1899733

Title:
  Qemu 5.1.0-1 GPU passthrough - MacOS vm starts only if hdmi is
  attached

Status in QEMU:
  New

Bug description:
  Hi,
  I have a macOS 10.15.7 vm with GPU passthrough (NVIDIA GTX Titan Black), 
libvirt xml has path to vbios too.
  Qemu 5.1.0-1 with libvirt 6.5.0-2 are installed in manjaro architect 20.1.1 
(two kernels tried: 5.4.67 LTS and 5.8.11 stable, no difference).
  I have two monitors, one with hdmi and one with vga inputs.
  Usually the gpu is connected to one monitor with hdmi, the other one with DVI 
(on gpu)-->vga adapter, so:
  1st monitor: hdmi-->hdmi
  2nd monitor: DVI-->vga adapter-->vga

  With this setup, launching "virsh start Catalina" has no problem.

  If I detach the hdmi cable from monitor 1, I cannot start qemu anymore: the 
2nd monitor turns black, it doesn't seem it has "no-signal", but only a black 
screen with the power led blinking (usually a window on the monitor floats 
around with "no signal" displayed when there is no signal to monitor).
  I say "qemu doesn't start" because in /var/log/libvirt/qemu/Catalina.log 
there's no trace.

  Detaching hdmi works with qemu 4.2 and libvirt 5.10, so this could be
  related to qemu update.

  Apologize, I know there aren't much information here, if someone can
  guide me to test the issue I would be grateful.

  Thanks for your attention

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