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[vfio-users] Addition of machine flag vmport=off causes VM hang (100% CPU usage) when AMD driver loads

Hi all,

I've had BIOS Q35-based Windows 7 guest with a Radeon R9 270X GPU passthrough working very well with F21, but recently I upgraded to F23 and also needed to reformat my guest so I started from scratch.

Using virt-manager I created a new BIOS Q35 machine using the <emulator> script wrapper trick from the VFIO series part 5 [1], configured my pass-through devices and booted it up. VGA output worked well, but *only* using the Windows driver; using AMD's drivers (either Catalyst or Crimson) caused the VM to hang on a black screen with 100% guest CPU usage just after the Windows logo splashscreen at boot disappeared - I presume as soon as the AMD driver initialized. Safe mode or startup repair remained functional.

My old VM continued to work, so after lots and lots of config testing, I found that despite many differences I thought would matter between the configurations (machine type q35 1.5 vs 2.5, HyperV extensions, differing PCI IDs, toggling HPET), it was actually virt-manager's addition of the vmport=off machine flag that caused the hangs. After removing the flag, the guest now works well with AMD's driver installed. Interestingly, while troubleshooting the issue I sometimes needed to reboot the host to get things working again after a failed boot - i.e. my previous configuration, which worked upon clean boot, would also start to fail if I had tried booting a configuration with vmport=off first. With the flag removed, I've been able to reboot multiple times in a row with no issues.

Since the packages in F23 virt-preview now support the "ioh3420" pcie-root-port device, I'm very happy that it's now possible to use a fully native configuration without the need for manual QEMU args. Along with the removal of vmport=off and setting <emulator> (to patch in x-vga=on to the command line), things are working well.

Additional information:

* CPU: Xeon E3-1225 v3
* GPU: AMD Radeon R9 270X
* Motherboard: Supermicro X10SAE

* OS: Fedora 23 + virt-preview
* libvirt: 1.3.0-1.fc23.x86_64
* qemu: 2.5.0-1.fc23.x86_64
* kernel: 4.2.8-300.fc23.x86_64

My issues were very similar to this user's report, who I see also has vmport=off in the CLI flags:

[1] http://vfio.blogspot.ca/2015/05/vfio-gpu-how-to-series-part-5-vga-mode.html


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