pve-docs/qm-pci-passthrough.adoc

[[qm_pci_passthrough]]
PCI(e) Passthrough
------------------
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:pve-toplevel:
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PCI(e) passthrough is a mechanism to give a virtual machine control over
a PCI device from the host. This can have some advantages over using
virtualized hardware, for example lower latency, higher performance, or more
features (e.g., offloading).

But, if you pass through a device to a virtual machine, you cannot use that
device anymore on the host or in any other VM.

Note that, while PCI passthrough is available for i440fx and q35 machines, PCIe
passthrough is only available on q35 machines. This does not mean that
PCIe capable devices that are passed through as PCI devices will only run at
PCI speeds. Passing through devices as PCIe 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.

General Requirements
~~~~~~~~~~~~~~~~~~~~

Since passthrough is performed on real hardware, it needs to fulfill some
requirements. A brief overview of these requirements is given below, for more
information on specific devices, see
https://pve.proxmox.com/wiki/PCI_Passthrough[PCI Passthrough Examples].

Hardware
^^^^^^^^
Your hardware needs to support `IOMMU` (*I*/*O* **M**emory **M**anagement
**U**nit) interrupt remapping, this includes the CPU and the motherboard.

Generally, Intel systems with VT-d and AMD systems with AMD-Vi support this.
But it is not guaranteed that everything will work out of the box, due
to bad hardware implementation and missing or low quality drivers.

Further, server grade hardware has often better support than consumer grade
hardware, but even then, many modern system can support this.

Please refer to your hardware vendor to check if they support this feature
under Linux for your specific setup.

Determining PCI Card Address
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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.

You can locate your card using

----
 lspci
----

Configuration
^^^^^^^^^^^^^

Once you ensured that your hardware supports passthrough, you will need to do
some configuration to enable PCI(e) passthrough.

.IOMMU

You will have to enable IOMMU support in your BIOS/UEFI. Usually the
corresponding setting is called `IOMMU` or `VT-d`, but you should find the exact
option name in the manual of your motherboard.

With AMD CPUs IOMMU is enabled by default. With recent kernels (6.8 or newer),
this is also true for Intel CPUs. On older kernels, it is necessary to enable
it on Intel CPUs via the
xref:sysboot_edit_kernel_cmdline[kernel command line] by adding:

----
 intel_iommu=on
----

.IOMMU Passthrough Mode

If your hardware supports IOMMU passthrough mode, enabling this mode might
increase performance.
This is because VMs then bypass the (default) DMA translation normally
performed by the hyper-visor and instead pass DMA requests directly to the
hardware IOMMU. To enable these options, add:

----
 iommu=pt
----

to the xref:sysboot_edit_kernel_cmdline[kernel commandline].

.Kernel Modules

You have to make sure the following modules are loaded. This can be achieved by
adding them to `'/etc/modules''.

.Mediated devices passthrough
[NOTE]
====
If passing through mediated devices (e.g. vGPUs), the following is not needed.
In these cases, the device will be owned by the appropriate host-driver
directly.
====

----
 vfio
 vfio_iommu_type1
 vfio_pci
----

[[qm_pci_passthrough_update_initramfs]]
After changing anything modules related, you need to refresh your
`initramfs`. On {pve} this can be done by executing:

----
# update-initramfs -u -k all
----

To check if the modules are being loaded, the output of

----
# lsmod | grep vfio
----

should include the four modules from above.

.Finish Configuration

Finally reboot to bring the changes into effect and check that it is indeed
enabled.

----
# dmesg | grep -e DMAR -e IOMMU -e AMD-Vi
----

should display that `IOMMU`, `Directed I/O` or `Interrupt Remapping` is
enabled, depending on hardware and kernel the exact message can vary.

For notes on how to troubleshoot or verify if IOMMU is working as intended, please
see the https://pve.proxmox.com/wiki/PCI_Passthrough#Verifying_IOMMU_parameters[Verifying IOMMU Parameters]
section in our wiki.

It is also important that the device(s) you want to pass through
are in a *separate* `IOMMU` group. This can be checked with a call to the {pve}
API:

----
# pvesh get /nodes/{nodename}/hardware/pci --pci-class-blacklist ""
----

It is okay if the device is in an `IOMMU` group together with its functions
(e.g. a GPU with the HDMI Audio device) or with its root port or PCI(e) bridge.

.PCI(e) slots
[NOTE]
====
Some platforms handle their physical PCI(e) slots differently. So, sometimes
it can help to put the card in a another PCI(e) slot, if you do not get the
desired `IOMMU` group separation.
====

.Unsafe interrupts
[NOTE]
====
For some platforms, it may be necessary to allow unsafe interrupts.
For this add  the following line in a file ending with `.conf' file in
*/etc/modprobe.d/*:

----
 options vfio_iommu_type1 allow_unsafe_interrupts=1
----

Please be aware that this option can make your system unstable.
====

GPU Passthrough Notes
^^^^^^^^^^^^^^^^^^^^^

It is not possible to display the frame buffer of the GPU via NoVNC or SPICE on
the {pve} web interface.

When passing through a whole GPU or a vGPU and graphic output is wanted, one
has to either physically connect a monitor to the card, or configure a remote
desktop software (for example, VNC or RDP) inside the guest.

If you want to use the GPU as a hardware accelerator, for example, for
programs using OpenCL or CUDA, this is not required.

Host Device Passthrough
~~~~~~~~~~~~~~~~~~~~~~~

The most used variant of PCI(e) passthrough is to pass through a whole
PCI(e) card, for example a GPU or a network card.


Host Configuration
^^^^^^^^^^^^^^^^^^

{pve} tries to automatically make the PCI(e) device unavailable for the host.
However, if this doesn't work, there are two things that can be done:

* pass the device IDs to the options of the 'vfio-pci' modules by adding
+
----
 options vfio-pci ids=1234:5678,4321:8765
----
+
to a .conf file in */etc/modprobe.d/* where `1234:5678` and `4321:8765` are
the vendor and device IDs obtained by:
+
----
# lspci -nn
----

* blacklist the driver on the host completely, ensuring that it is free to bind
for passthrough, with
+
----
 blacklist DRIVERNAME
----
+
in a .conf file in */etc/modprobe.d/*.
+
To find the drivername, execute
+
----
# lspci -k
----
+
for example:
+
----
# lspci -k | grep -A 3 "VGA"
----
+
will output something similar to
+
----
01:00.0 VGA compatible controller: NVIDIA Corporation GP108 [GeForce GT 1030] (rev a1)
	Subsystem: Micro-Star International Co., Ltd. [MSI] GP108 [GeForce GT 1030]
	Kernel driver in use: <some-module>
	Kernel modules: <some-module>
----
+
Now we can blacklist the drivers by writing them into a .conf file:
+
----
echo "blacklist <some-module>" >> /etc/modprobe.d/blacklist.conf
----

For both methods you need to
xref:qm_pci_passthrough_update_initramfs[update the `initramfs`] again and
reboot after that.

Should this not work, you might need to set a soft dependency to load the gpu
modules before loading 'vfio-pci'. This can be done with the 'softdep' flag, see
also the manpages on 'modprobe.d' for more information.

For example, if you are using drivers named <some-module>:

----
# echo "softdep <some-module> pre: vfio-pci" >> /etc/modprobe.d/<some-module>.conf
----


.Verify Configuration

To check if your changes were successful, you can use

----
# lspci -nnk
----

and check your device entry. If it says

----
Kernel driver in use: vfio-pci
----

or the 'in use' line is missing entirely, the device is ready to be used for
passthrough.

.Mediated devices
[NOTE]
====
For mediated devices this line will differ as the device will be owned as the
host driver directly, not 'vfio-pci'.
====

[[qm_pci_passthrough_vm_config]]
VM Configuration
^^^^^^^^^^^^^^^^
When passing through a GPU, the best compatibility is reached when using
'q35' as machine type, 'OVMF' ('UEFI' for VMs) instead of SeaBIOS and PCIe
instead of PCI. Note that if you want to use 'OVMF' for GPU passthrough, the
GPU needs to have an UEFI capable ROM, otherwise use SeaBIOS instead. To check if
the ROM is UEFI capable, see the
https://pve.proxmox.com/wiki/PCI_Passthrough#How_to_know_if_a_graphics_card_is_UEFI_.28OVMF.29_compatible[PCI Passthrough Examples]
wiki.

Furthermore, using OVMF, disabling vga arbitration may be possible, reducing the
amount of legacy code needed to be run during boot. To disable vga arbitration:

----
 echo "options vfio-pci ids=<vendor-id>,<device-id> disable_vga=1" > /etc/modprobe.d/vfio.conf
----

replacing the <vendor-id> and <device-id> with the ones obtained from:

----
# lspci -nn
----

PCI devices can be added in the web interface in the hardware section of the VM.
Alternatively, you can use the command line; set the *hostpciX* option in the VM
configuration, for example by executing:

----
# qm set VMID -hostpci0 00:02.0
----

or by adding a line to the VM configuration file:

----
 hostpci0: 00:02.0
----


If your device has multiple functions (e.g., ``00:02.0`' and ``00:02.1`' ),
you can pass them through all together with the shortened syntax ``00:02`'.
This is equivalent with checking the ``All Functions`' checkbox in the
web interface.

There are some options to which may be necessary, depending on the device
and guest OS:

* *x-vga=on|off* marks the PCI(e) device as the primary GPU of the VM.
With this enabled the *vga* configuration option will be ignored.

* *pcie=on|off* tells {pve} to use a PCIe or PCI port. Some guests/device
combination require PCIe rather than PCI. PCIe is only available for 'q35'
machine types.

* *rombar=on|off* makes the firmware ROM visible for the guest. Default is on.
Some PCI(e) devices need this disabled.

* *romfile=<path>*, is an optional path to a ROM file for the device to use.
This is a relative path under */usr/share/kvm/*.

.Example

An example of PCIe passthrough with a GPU set to primary:

----
# qm set VMID -hostpci0 02:00,pcie=on,x-vga=on
----

.PCI ID overrides

You can override the PCI vendor ID, device ID, and subsystem IDs that will be
seen by the guest. This is useful if your device is a variant with an ID that
your guest's drivers don't recognize, but you want to force those drivers to be
loaded anyway (e.g. if you know your device shares the same chipset as a
supported variant).

The available options are `vendor-id`, `device-id`, `sub-vendor-id`, and
`sub-device-id`. You can set any or all of these to override your device's
default IDs.

For example:

----
# qm set VMID -hostpci0 02:00,device-id=0x10f6,sub-vendor-id=0x0000
----

SR-IOV
~~~~~~

Another variant for passing through PCI(e) devices is to use the hardware
virtualization features of your devices, if available.

.Enabling SR-IOV
[NOTE]
====
To use SR-IOV, platform support is especially important. It may be necessary
to enable this feature in the BIOS/UEFI first, or to use a specific PCI(e) port
for it to work. In doubt, consult the manual of the platform or contact its
vendor.
====

'SR-IOV' (**S**ingle-**R**oot **I**nput/**O**utput **V**irtualization) enables
a single device to provide multiple 'VF' (**V**irtual **F**unctions) to the
system. Each of those 'VF' can be used in a different VM, with full hardware
features and also better performance and lower latency than software
virtualized devices.

Currently, the most common use case for this are NICs (**N**etwork
**I**nterface **C**ard) with SR-IOV support, which can provide multiple VFs per
physical port. This allows using features such as checksum offloading, etc. to
be used inside a VM, reducing the (host) CPU overhead.

Host Configuration
^^^^^^^^^^^^^^^^^^

Generally, there are two methods for enabling virtual functions on a device.

* sometimes there is an option for the driver module e.g. for some
Intel drivers
+
----
 max_vfs=4
----
+
which could be put file with '.conf' ending under */etc/modprobe.d/*.
(Do not forget to update your initramfs after that)
+
Please refer to your driver module documentation for the exact
parameters and options.

* The second, more generic, approach is using the `sysfs`.
If a device and driver supports this you can change the number of VFs on
the fly. For example, to setup 4 VFs on device 0000:01:00.0 execute:
+
----
# echo 4 > /sys/bus/pci/devices/0000:01:00.0/sriov_numvfs
----
+
To make this change persistent you can use the `sysfsutils` Debian package.
After installation configure it via */etc/sysfs.conf* or a `FILE.conf' in
*/etc/sysfs.d/*.

VM Configuration
^^^^^^^^^^^^^^^^

After creating VFs, you should see them as separate PCI(e) devices when
outputting them with `lspci`. Get their ID and pass them through like a
xref:qm_pci_passthrough_vm_config[normal PCI(e) device].

Mediated Devices (vGPU, GVT-g)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Mediated devices are another method to reuse features and performance from
physical hardware for virtualized hardware. These are found most common in
virtualized GPU setups such as Intel's GVT-g and NVIDIA's vGPUs used in their
GRID technology.

With this, a physical Card is able to create virtual cards, similar to SR-IOV.
The difference is that mediated devices do not appear as PCI(e) devices in the
host, and are such only suited for using in virtual machines.

Host Configuration
^^^^^^^^^^^^^^^^^^

In general your card's driver must support that feature, otherwise it will
not work. So please refer to your vendor for compatible drivers and how to
configure them.

Intel's drivers for GVT-g are integrated in the Kernel and should work
with 5th, 6th and 7th generation Intel Core Processors, as well as E3 v4, E3
v5 and E3 v6 Xeon Processors.

To enable it for Intel Graphics, you have to make sure to load the module
'kvmgt' (for example via `/etc/modules`) and to enable it on the
xref:sysboot_edit_kernel_cmdline[Kernel commandline] and add the following parameter:

----
 i915.enable_gvt=1
----

After that remember to
xref:qm_pci_passthrough_update_initramfs[update the `initramfs`],
and reboot your host.

VM Configuration
^^^^^^^^^^^^^^^^

To use a mediated device, simply specify the `mdev` property on a `hostpciX`
VM configuration option.

You can get the supported devices via the 'sysfs'. For example, to list the
supported types for the device '0000:00:02.0' you would simply execute:

----
# ls /sys/bus/pci/devices/0000:00:02.0/mdev_supported_types
----

Each entry is a directory which contains the following important files:

* 'available_instances' contains the amount of still available instances of
this type, each 'mdev' use in a VM reduces this.
* 'description' contains a short description about the capabilities of the type
* 'create' is the endpoint to create such a device, {pve} does this
automatically for you, if a 'hostpciX' option with `mdev` is configured.

Example configuration with an `Intel GVT-g vGPU` (`Intel Skylake 6700k`):

----
# qm set VMID -hostpci0 00:02.0,mdev=i915-GVTg_V5_4
----

With this set, {pve} automatically creates such a device on VM start, and
cleans it up again when the VM stops.

Use in Clusters
~~~~~~~~~~~~~~~

It is also possible to map devices on a cluster level, so that they can be
properly used with HA and hardware changes are detected and non root users
can configure them. See xref:resource_mapping[Resource Mapping]
for details on that.

[[qm_pci_viommu]]
vIOMMU (emulated IOMMU)
~~~~~~~~~~~~~~~~~~~~~~~

vIOMMU is the emulation of a hardware IOMMU within a virtual machine, providing
improved memory access control and security for virtualized I/O devices. Using
the vIOMMU option also allows you to pass through PCI(e) devices to level-2 VMs
in level-1 VMs via
https://pve.proxmox.com/wiki/Nested_Virtualization[Nested Virtualization].
To pass through physical PCI(e) devices from the host to nested VMs, follow the
PCI(e) passthrough instructions.

There are currently two vIOMMU implementations available: Intel and VirtIO.

Intel vIOMMU
^^^^^^^^^^^^

Intel vIOMMU specific VM requirements:

* Whether you are using an Intel or AMD CPU on your host, it is important to set
`intel_iommu=on` in the VMs kernel parameters.

* To use Intel vIOMMU you need to set *q35* as the machine type.

If all requirements are met, you can add `viommu=intel` to the machine parameter
in the configuration of the VM that should be able to pass through PCI devices.

----
# qm set VMID -machine q35,viommu=intel
----

https://wiki.qemu.org/Features/VT-d[QEMU documentation for VT-d]

VirtIO vIOMMU
^^^^^^^^^^^^^

This vIOMMU implementation is more recent and does not have as many limitations
as Intel vIOMMU but is currently less used in production and less documentated.

With VirtIO vIOMMU there is *no* need to set any kernel parameters. It is also
*not* necessary to use q35 as the machine type, but it is advisable if you want
to use PCIe.

----
# qm set VMID -machine q35,viommu=virtio
----

https://web.archive.org/web/20230804075844/https://michael2012z.medium.com/virtio-iommu-789369049443[Blog-Post by Michael Zhao explaining virtio-iommu]

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See Also
~~~~~~~~

* link:/wiki/Pci_passthrough[PCI Passthrough Examples]

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