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I recently installed SteamOS via libvirt and kvm. I am using the latest install media based off of Debian Jessie. I know that pci passthrough works on my hardware because I have a windows guest already configured. My card is a gtx 980 ti and the host is using onboard intel graphics. Cpu is i5-4690k. When I configure the passthrough it freezes on the ovmf loader screen. I am following the wiki guide here pretty much to the letter. I have tried both i440fx and q35 chipset. Interestingly, on the i440fx chip I couldn't even get the vm to boot without passthrough on a virtual screen so I have gotten farther with q35 and that's what I am working on now. Below is my current libvirt xml.
<domain type='kvm'>
<name>debian8</name>
<uuid>b3abb269-4e30-4e05-9624-bfd4b3fab371</uuid>
<memory unit='KiB'>8388608</memory>
<currentMemory unit='KiB'>8388608</currentMemory>
<vcpu placement='static'>4</vcpu>
<os>
<type arch='x86_64' machine='pc-q35-2.7'>hvm</type>
<loader readonly='yes' type='pflash'>/usr/share/ovmf/x64/ovmf_x64.bin</loader>
<nvram>/var/lib/libvirt/qemu/nvram/debian8_VARS.fd</nvram>
<bootmenu enable='yes'/>
</os>
<features>
<acpi/>
<apic/>
<kvm>
<hidden state='on'/>
</kvm>
<vmport state='off'/>
</features>
<cpu mode='host-passthrough'>
<topology sockets='1' cores='4' threads='1'/>
</cpu>
<clock offset='utc'>
<timer name='rtc' tickpolicy='catchup'/>
<timer name='pit' tickpolicy='delay'/>
<timer name='hpet' present='no'/>
</clock>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<pm>
<suspend-to-mem enabled='no'/>
<suspend-to-disk enabled='no'/>
</pm>
<devices>
<emulator>/usr/sbin/qemu-system-x86_64</emulator>
<disk type='block' device='disk'>
<driver name='qemu' type='raw' cache='none' io='native'/>
<source dev='/dev/OS/debian8'/>
<target dev='sda' bus='scsi'/>
<boot order='1'/>
<address type='drive' controller='0' bus='0' target='0' unit='0'/>
</disk>
<controller type='usb' index='0' model='ich9-ehci1'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1d' function='0x7'/>
</controller>
<controller type='usb' index='0' model='ich9-uhci1'>
<master startport='0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1d' function='0x0' multifunction='on'/>
</controller>
<controller type='usb' index='0' model='ich9-uhci2'>
<master startport='2'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1d' function='0x1'/>
</controller>
<controller type='usb' index='0' model='ich9-uhci3'>
<master startport='4'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1d' function='0x2'/>
</controller>
<controller type='scsi' index='0' model='virtio-scsi'>
<address type='pci' domain='0x0000' bus='0x02' slot='0x03' function='0x0'/>
</controller>
<controller type='sata' index='0'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1f' function='0x2'/>
</controller>
<controller type='pci' index='0' model='pcie-root'/>
<controller type='pci' index='1' model='dmi-to-pci-bridge'>
<model name='i82801b11-bridge'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1e' function='0x0'/>
</controller>
<controller type='pci' index='2' model='pci-bridge'>
<model name='pci-bridge'/>
<target chassisNr='2'/>
<address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
</controller>
<interface type='network'>
<mac address='52:54:00:e7:a6:c2'/>
<source network='default'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x02' slot='0x01' function='0x0'/>
</interface>
<input type='mouse' bus='ps2'/>
<input type='keyboard' bus='ps2'/>
<hostdev mode='subsystem' type='pci' managed='yes'>
<source>
<address domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
</source>
<address type='pci' domain='0x0000' bus='0x02' slot='0x02' function='0x0'/>
</hostdev>
<hostdev mode='subsystem' type='pci' managed='yes'>
<source>
<address domain='0x0000' bus='0x01' slot='0x00' function='0x1'/>
</source>
<address type='pci' domain='0x0000' bus='0x02' slot='0x04' function='0x0'/>
</hostdev>
<hostdev mode='subsystem' type='pci' managed='yes'>
<source>
<address domain='0x0000' bus='0x00' slot='0x1b' function='0x0'/>
</source>
<address type='pci' domain='0x0000' bus='0x02' slot='0x06' function='0x0'/>
</hostdev>
<memballoon model='virtio'>
<address type='pci' domain='0x0000' bus='0x02' slot='0x05' function='0x0'/>
</memballoon>
</devices>
</domain>
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