Pci passthrough

From Proxmox VE
Jump to: navigation, search


PCI passthrough allows you to use a physical PCI device (graphics card, network card) inside a VM (KVM virtualization only).

If you "PCI passthrough" a device, the device is not available to the host anymore.


PCI passthrough is an experimental feature in Proxmox VE

Enable the IOMMU

You need to enable the IOMMU, by editing the kernel commandline.

First open your bootloader kernel command line config file, for grub:

nano /etc/default/grub

or systemd-boot

nano /etc/kernel/cmdline

Find the line with "GRUB_CMDLINE_LINUX_DEFAULT" (for GRUB), create the file for systemd-boot (it'S format is a single line with options)

Intel CPU

For Intel CPUs add "intel_iommu=on", for example:

GRUB_CMDLINE_LINUX_DEFAULT="quiet intel_iommu=on" 

Safe the changes and update grub:


or: pve-efiboot-tool refresh

Then reboot, after that run "dmesg | grep -e DMAR -e IOMMU" from the command line. If there is no output, then something is wrong.


For AMD CPUs add "amd_iommu=on", for example:

GRUB_CMDLINE_LINUX_DEFAULT="quiet amd_iommu=on" 

Safe the changes and update grub:



pve-efiboot-tool refresh

Then reboot, after that run "dmesg | grep -e DMAR -e IOMMU" from the command line. If there is no output, then something is wrong.

PT Mode

Both Intel and AMD chips can use the additional parameter "iommu=pt", added in the same way as above.

This enables the IOMMU translation only when necessary, and can thus improve performance for PCIe devices not used in VMs.

Required Modules

add to /etc/modules


Note that in the 5.4 based kernel (will be used for Proxmox VE 6.2 in Q2/2020) some of those modules are already built into the kernel directly.

IOMMU Interrupt Remapping

It will not be possible to use PCI passthrough without interrupt remapping. Device assignment will fail with 'Failed to assign device "[device name]": Operation not permitted' or 'Interrupt Remapping hardware not found, passing devices to unprivileged domains is insecure.' error.

All systems using an Intel processor and chipset that have support for Intel Virtualization Technology for Directed I/O (VT-d), but do not have support for interrupt remapping will see such an error. Interrupt remapping support is provided in newer processors and chipsets (both AMD and Intel).

To identify if your system has support for interrupt remapping:

dmesg | grep 'remapping'

If you see one of the following lines:

  • "AMD-Vi: Interrupt remapping enabled"
  • "DMAR-IR: Enabled IRQ remapping in x2apic mode" ('x2apic' can be different on old CPUs, but should still work)

then remapping is supported.

If your system doesn't support interrupt remapping, you can allow unsafe interrupts with:

echo "options vfio_iommu_type1 allow_unsafe_interrupts=1" > /etc/modprobe.d/iommu_unsafe_interrupts.conf

Verify IOMMU Isolation

For working PCI passthrough, you need a dedicated IOMMU group for all PCI devices you want to assign to a VM.

You should have something like:

# find /sys/kernel/iommu_groups/ -type l

To have separate IOMMU groups, your processor needs to have support for a feature called ACS (Access Control Services).

All Xeon processor support them (E3,E5) excluding Xeon E3-1200.

For Intel Core it's different, only some processors support ACS. Anything newer than listed below should support ACS, as long as VT-d is supported. See https://ark.intel.com for more info.

Haswell-E (LGA2011-v3)
i7-5960X (8-core, 3/3.5GHz)
i7-5930K (6-core, 3.2/3.8GHz)
i7-5820K (6-core, 3.3/3.6GHz)

Ivy Bridge-E (LGA2011)
i7-4960X (6-core, 3.6/4GHz)
i7-4930K (6-core, 3.4/3.6GHz)
i7-4820K (4-core, 3.7/3.9GHz)

Sandy Bridge-E (LGA2011)
i7-3960X (6-core, 3.3/3.9GHz)
i7-3970X (6-core, 3.5/4GHz)
i7-3930K (6-core, 3.2/3.8GHz)
i7-3820 (4-core, 3.6/3.8GHz)

AMD chips from Ryzen 1st generation and newer are fine too.

If you don't have dedicated IOMMU groups, you can try:

1) moving the card to another pci slot

2) adding "pcie_acs_override=downstream" to kernel boot commandline (grub or systemd-boot) options, which can help on some setup with bad ACS implementation.

Checkout the documentation about Editing the kernel commandline

More infos:

http://vfio.blogspot.be/2015/10/intel-processors-with-acs-support.html http://vfio.blogspot.be/2014/08/iommu-groups-inside-and-out.html

Determine your PCI card address, and configure your VM

The easiest way is to use the GUI to add a device of type "Host PCI" in the VM's hardware tab.

Alternatively, you can use the command line:

Locate your card using "lspci". The address should be in the form of: 01:00.0 Edit the <vmid>.conf file. It can be located at: /etc/pve/qemu-server/vmid.conf.

Add this line to the end of the file:

hostpci0: 01:00.0

If you have a multi-function device (like a vga card with embedded audio chipset), you can pass all functions manually with:

hostpci0: 01:00.0;01:00.1

or, to pass all functions automatically:

hostpci0: 01:00

PCI Express Passthrough

Check the "PCI-E" checkbox in the GUI when adding your device, or manually add the pcie=1 parameter to your VM config:

machine: q35
hostpci0: 01:00.0,pcie=1

PCIe passthrough is only supported on Q35 machines.

Note that this does not mean that devices assigned without this setting will only have PCI speeds, it just sets a flag for the guest to tell it that the device is a PCIe device instead of a "really-fast legacy PCI device". Some guest applications benefit from this.

GPU Passthrough

Yellowpin.svg Note: See http://blog.quindorian.org/2018/03/building-a-2u-amd-ryzen-server-proxmox-gpu-passthrough.html/ if you like an article with a HOWTO approach.
  • AMD RADEON 5xxx, 6xxx, 7xxx, Navi 5XXX(XT), NVIDIA GEFORCE 7, 8, GTX 4xx, 5xx, 6xx, 7xx, 9xx, 10xx and RTX 16xx/20xx have been reported working.
  • You might need to load some specific options in grub.cfg or other tuning values to get your configuration specifically working/stable
  • Here's a good forum thread of archlinux: https://bbs.archlinux.org/viewtopic.php?id=162768

For a GPU, it's often helpful if the host doesn't try to use the GPU, which avoids issues with the host driver unbinding and re-binding to the device. Sometimes making sure the host BIOS POST messages are displayed on a different GPU is helpful too. This can sometimes be acomplished via BIOS settings, moving the card to a different slot or enabling/disabling legacy boot support.

First, find the device and vendor id of your vga card:

$ lspci -n -s 01:00
01:00.0 0300: 10de:1381 (rev a2)
01:00.1 0403: 10de:0fbc (rev a1)

The Vendor:Device IDs for this GPU and it's audio functions are therefore 10de:1381, 10de:0fbc.

Then, create a file:

echo "options vfio-pci ids=10de:1381,10de:0fbc" > /etc/modprobe.d/vfio.conf

blacklist the drivers:

echo "blacklist radeon" >> /etc/modprobe.d/blacklist.conf 
echo "blacklist nouveau" >> /etc/modprobe.d/blacklist.conf 
echo "blacklist nvidia" >> /etc/modprobe.d/blacklist.conf 

and reboot your machine.

For VM configuration, They are 4 configurations possible:

GPU OVMF PCI Passthrough (recommended)

Select "OVMF" as "BIOS" for your VM instead of the default "SeaBIOS". You need to install your guest OS with uefi support. (for Windows, try win >=8)

Using OVMF, you can also add disable_vga=1 to vfio-pci module, which try to to opt-out devices from vga arbitration if possible:

echo "options vfio-pci ids=10de:1381,10de:0fbc disable_vga=1" > /etc/modprobe.d/vfio.conf

and you need to make sure your graphics card has an UEFI bootable rom: http://vfio.blogspot.fr/2014/08/does-my-graphics-card-rom-support-efi.html

bios: ovmf
scsihw: virtio-scsi-pci
bootdisk: scsi0
scsi0: .....
hostpci0: 01:00,x-vga=on

GPU OVMF PCI Express Passthrough

Same as above, but set machine type to q35 and enable pcie=1:

bios: ovmf
scsihw: virtio-scsi-pci
bootdisk: scsi0
scsi0: .....
machine: q35
hostpci0: 01:00,pcie=1,x-vga=on

GPU Seabios PCI Passthrough

hostpci0: 01:00,x-vga=on

GPU Seabios PCI Express Passthrough

machine: q35
hostpci0: 01:00,pcie=1,x-vga=on

How to know if a Graphics Card is UEFI (OVMF) compatible

Get and compile the software "rom-parser":

$ git clone https://github.com/awilliam/rom-parser
$ cd rom-parser
$ make

Then dump the rom of you vga card:

# cd /sys/bus/pci/devices/0000:01:00.0/
# echo 1 > rom
# cat rom > /tmp/image.rom
# echo 0 > rom

and test it with:

./rom-parser /tmp/image.rom

Valid ROM signature found @0h, PCIR offset 190h
 PCIR: type 0, vendor: 10de, device: 1280, class: 030000
 PCIR: revision 0, vendor revision: 1
Valid ROM signature found @f400h, PCIR offset 1ch
 PCIR: type 3, vendor: 10de, device: 1280, class: 030000
 PCIR: revision 3, vendor revision: 0
  EFI: Signature Valid
 Last image

To be UEFI compatible, you need a "type 3" in the result.


Some Windows applications like geforce experience, Passmark Performance Test and SiSoftware Sandra crash can crash the VM. You need to add:

echo "options kvm ignore_msrs=1" > /etc/modprobe.d/kvm.conf

If you see a lot of warning messages in your 'dmesg' system log, add the following instead:

echo "options kvm ignore_msrs=1 report_ignored_msrs=0" > /etc/modprobe.d/kvm.conf

User have reported that NVIDIA Kepler K80 GPUs need this in vmid.conf:

args: -machine pc,max-ram-below-4g=1G

The 'romfile' Option


Some motherboards can't passthrough GPUs on the first PCI(e) slot by default, because its vbios is shadowed during bootup. You need to capture its vBIOS when its working "normally" (i.e. installed in a different slot), then you can move the card to slot 1 and start the vm using the dumped vBIOS.

To dump the bios:

cd /sys/bus/pci/devices/0000:01:00.0/
echo 1 > rom
cat rom > /usr/share/kvm/vbios.bin
echo 0 > rom

Then you can pass the vbios file (must be located in /usr/share/kvm/) with:

hostpci0: 01:00,x-vga=on,romfile=vbios.bin


BAR 3: can't reserve [mem] error

If you have this error when you try to use the card for a VM:

vfio-pci 0000:04:00.0: BAR 3: can't reserve [mem 0xca000000-0xcbffffff 64bit]

you can try to add the following kernel commandline option:


Checkout the documentation about Editing the kernel commandline


Spice may give trouble when passing through a GPU as it presents a "virtual" PCI graphic card to the guest and some drivers have problems with that, even when both cards show up. It's always worth a try to disable SPICE and check again if something fails.

HDMI Audio crackling/broken

Some digital audio devices (usually added via GPU functions) may require MSI (Message Signaled Interrupts) to be enabled to function correctly. If you experience any issues, try changing MSI settings in the guest and rebooting the guest.

A Windows-Tool to simplify this is available here: https://github.com/CHEF-KOCH/MSI-utility/releases/latest

Linux guests usually enable MSI by themselves. To force use of MSI for GPU audio devices, use the following command and reboot:

echo "options snd-hda-intel enable_msi=1" >> /etc/modprobe.d/snd-hda-intel.conf

Use 'lspci -vv' and check for the following line on your device to see if MSI is enabled:

Capabilities: [a0] MSI: Enable+ Count=1/1 Maskable- 64bit+

If it says 'Enable+', MSI is working, 'Enable-' means it is supported but disabled, and if the line is missing, MSI is not supported by the PCIe hardware.

This can potentially also improve performance for other passthrough devices, including GPUs, but that depends on the hardware being used.

BIOS options

Make sure you are using the most recent BIOS version for you mainboard. Often IOMMU groupings or passthrough support in general is improved in later versions.

Some general BIOS options that might need changing to allow passthrough to work:

  • IOMMU or VT-d: Set to 'Enabled' or equivalent, often 'Auto' is not the same
  • 'Legacy boot' or CSM: For GPU passthrough it can help to disable this, but keep in mind that PVE has to be installed in UEFI mode, as it will not boot in BIOS mode without this enabled. The reason for disabling this is that it avoids legacy VGA initialization of installed GPUs, making them able to be re-initialized later, as required for passthrough. Most useful when trying to use passthrough in single GPU systems.

Verify Operation

Start the VM and enter the qm monitor onn the CLI: "qm monitor vmnumber" Verify that your card is listed here: "info pci" Then install drivers on your guest OS.

NOTE: Card support might be limited to 2 or 3 devices.

NOTE: A PCI device can only ever be attached to a single VM.

NOTE: This process will remove the card from the proxmox host OS as long as the VM it's attached to is running.

NOTE: Using PCI passthrough to present drives direct to a ZFS (FreeNAS, Openfiler, OmniOS) virtual machine is OK for testing, but not recommended for production use. Specific FreeNAS warnings can be found here: http://forums.freenas.org/threads/absolutely-must-virtualize-freenas-a-guide-to-not-completely-losing-your-data.12714/

USB Passthrough

If you need to passthrough usb devices (keyboard, mouse), please follow this wiki article: