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I'm currently running Arch with lxde DE & virt-manager on the host machine for a few Windows virtual machines. As I'm planning to run a macOS Catalina as another guest, it is advised to passthrough the iGPU to macOS guest for fully accelerated 3D because I don't have a discrete graphics card for now.
I wonder if I can directly boot into a guest OS headlessly on machine startup? And how do I get graphics back to Arch if something goes wrong under the hood?
ps: I'm using a Coffee-Lake-based CPU whose iGPU doesn't work with Intel gvt-g.
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I do not know virt-manager at all, so I can't give any answers on that. I would say, Look through the documentation for it and determine how to start VMs on startup.
I can tell you, this is possible with virtualbox. This page explains how to set up a headless VM under virtualbox, and this page here explains how to start a VM during system boot. It's not an exact fit for Arch, since arch uses systemd and not init, But it's a place to start.
tl;dr: It is possible, but you're going to have to do some research.
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You can connect via ssh to the arch server and use virsh start VM_NAME or mark it as autostart in virt-manager. Depending on the machine-setup there is vnc accessible using ports 590X (where X is depending on how much guests you are running), a display running on a tty or at least the virsh console VM_NAME command.
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