pass

Configure physical devices or data blob types passed through from the host to a guest

Synopsis:

[blob_type] pass [loc options] [intr guest_intr[,u][=vector_number]]

Options:

blob_type

The blob_type setting affects only the loc option. If blob_type is specified, then all locations specified by the loc options that follow (until the next context) provide data of the type blob_type to the guest that will be hosted in the VM being configured.

If blob_type precedes a load option, the qvm process copies the contents of the specified file into the guest system address space (see load in this chapter).

intr guest_intr[,u][=vector_number]

Pass the guest_intr interrupt through to the guest. This argument is associated with a host vector number, either vector_number if this value is specified, or the value specified by the hostvector vdev option for the PIC identified by guest_intr.

The host system uses an interrupt service thread (IST) to deliver guest_intr into the guest system. In this release, there's one thread for each passed-through interrupt.

The following attributes specify special behaviors:

u

The u attribute permits immediate unmasking of interrupts to a pass-through device, eliminating the need for a guest exit in order to unmask the EOI.

Warning:

This attribute may be used only if your system has been configured to meet certain conditions, including:

Interrupts in the hypervisor host are edge-triggered.
Interrupts in the guest are edge-triggered.
The guest's device driver won't create new interrupt conditions, including when it is clearing the interrupt.

Incorrect implementation of this feature might create a race condition and result in the loss of the device from the system. Contact your QNX representative before attempting to implement this feature.

loc [{mem:|io:}]region,length,{+|-|r|w|x|c|e|m|d|n|s|t}[,=host_location]

The host resource is made directly available to the guest starting at region and continuing for length bytes in guest-physical memory. The newly accessible region is typically memory, which is optionally prefixed with mem: (because this is the default resource type), but on the x86 you can prefix the location with io: to pass I/O space through.

The length argument is a 64-bit value. This can be followed by access type attributes, which may be a combination of the following symbols:


Argument	Description
`+`	Attributes following are added to the region (this is the initial state).
`-`	Attributes following are removed from the region.
`r`	Read
`w`	Write
`x`	Execute
`c`	Cachable
`e`	Report exception
`m`	Usable as system memory (implicitly specifies `+rwxcstn`). If you specify this attribute, you must not specify `d`.
`d`	Device uses DMA. If you're using smmuman to manage DMA, you must specify either this attribute or `n`. You may, however, choose to manage DMA without smmuman; for more information, read the note about the `d` semantics just below. If you specify `d`, you should specify at least the `r` and `w` access types.
`n`	Device doesn't use DMA. If you're using smmuman to manage DMA, you must specify either this attribute or `d`.
`s`	Region can be the source of a DMA request.
`t`	Region can be the target of a DMA request.

If no access types are specified, rw is assumed.

The last part of the loc mem: arguments string may be an equal sign followed by the host-physical address of the device on the system. For some but not all devices, you can provide the guest with an address that is different from the host-physical address (i.e., the device is at host location A, but the guest sees it at location B).

For example:

pass loc mem:0x2000,r=0x1000

specifies a read-only location at guest-physical address of 0x2000 for a device at the host-physical address of 0x1000.

CAUTION:

You can use the pass loc option to pass RAM (memory) through to a guest, but the RAM doesn't get zeroed, neither before the guest is given access to it, nor afterwards (e.g., after a VM termination). If you pass through RAM to which another guest system had access, remember that, unless you zero it, this memory may contain vestigial data from a previous guest system.

For the io: case, the location is a port number. For example:

io:4

specifies an x86 I/O device on port 4. You can provide a length and specify access type flags, but no host address is needed.

Note:

If you set the d attribute to specify that the device uses DMA, the qvm process by default expects to use the smmuman service to ensure that the DMA configuration is safe and secure. If this service isn't running, qvm outputs a fatal error message and aborts without starting the VM.

If, however, the smmu-externally-managed variable has been enabled (set to on), the d attribute has no effect and qvm outputs an information message saying so. Enabling this variable tells qvm to trust that the DMA configuration normally addressed by smmuman has been taken care of in some other way. Note that in this case, the VM configuration must not contain the smmu vdev or qvm reports a fatal error and aborts.

For more details about the smmu-externally-managed variable, refer to the Configuration variables reference.

loc pci:pci_vid/pci_did

The PCI device identified by the given vendor ID (pci_vid) and device ID (pci_did) is made available to the guest.

Note:

To support pass-through PCI devices, you must include the pci_server-qvm_support.so module in your hypervisor host image. For instructions on doing so, see the pci-server reference in the QNX OS Utilities Reference.

loc pci:pci_bus:pci_dev[.pci_func][=pci:pci_spec]

A PCI device is made available to the guest and placed at the guest PCI bus pci_bus, PCI device pci_dev, and PCI function pci_func if this field is given.

If no equals sign follows, the device being passed through is host pci_bus,pci_dev,pci_func. If there is an equals sign followed by another pci: prefix, then the pci_spec after this prefix is either pci_vid/pci_did or pci_bus:pci_dev[.pci_func], and it specifies the host vendor/device ID or the host bus/device/function to be passed through.

For example:

pass loc pci:0:12.0=pci:0x8086/0x1234

passes a device with vendor ID 0x8086 and device ID 0x1234 to the guest at bus 0, device 12, and function 0 (multi-function). At present, you may choose the device and function numbers in the guest, but the bus number must be 0.

Note:

Per the PCI specification, some multi-function device must implement function 0. If you want to make your PCI devices enumerable with non-zero function numbers, you can use the vdev-pci-dummy instance as a placeholder at function 0.

See Graphics device below for a configuration example that uses this location type.

sched priority

Set the priority of the pulses the host system uses to inform the qvm process instance of the pass-through interrupts for the current pass-through device (i.e., the one specified after this option in the current pass context).

For example, the following sets the pulse priority to 67 for interrupt 89 for the current device:

pass
    sched 67 intr gic:89

Similarly, the following sets the pulse priority to 44 for interrupts for the current device:

pass
    sched 44 loc pci:0:3.0

In this last example, no interrupt is specified because for PCI devices interrupts can be implicit (see Common vdev options in the Virtual Device Reference chapter).

For more information about scheduling priorities, see Scheduling in the Understanding QNX Virtual Environments chapter, and Thread scheduling in the QNX OS System Architecture guide.

Description:

The pass option specifies that one of the following should be passed through from the hypervisor host domain to the guest:

a physical device, such as a PCI audio device
a data blob, such as an initial RAM disk (initrd) for a Linux guest

Until the next pass, vdev, or reserve option is encountered in the configuration, all intr and loc options that follow a pass option specify interrupts and locations for this pass-through device or component.

CAUTION:

In general, only one OS may have direct control over a physical hardware resource such as a device or region of memory. In a system that supports virtualization, this means either the host OS or a guest OS (but not both) and requires important considerations when passing physical devices or memory through to a guest.

If the host owns a device that a guest requires pass-through access to, the host must terminate its driver for the device before the guest can start a driver for the device in its virtual environment. Similarly, if one guest owns a device as a pass-through device, it must terminate the device driver in its virtualized space before another guest can use the device in its space.

In short, you should never pass a DMA device through to more that one guest, and only in exceptional designs should you pass a non-DMA device through to more than one guest. If you believe that your design requires the use of a non-DMA device in this way, contact your QNX representative.

The same restriction applies to RAM. When you pass through host-physical memory to a guest, the host must have been configured, via options passed to the startup bootstrap program, to not allocate from that same memory region. Otherwise, it might try to use this memory and, thus, the host and guest OSs might end up corrupting each other's memory, leading to bad behavior or system crashes.

The qvm process recognizes the following data types (blob_type) specified before a pass option:

acpi

acpi pass

Only available for x86. Any loc options following will be interpreted as providing additional ACPI tables to the guest system.

data

Recognized, but not relevant for pass-through.

fdt

fdt pass

Any loc options following will be interpreted as providing an FDT binary blob to the guest system (see ACPI tables and FDTs in the Configuration chapter).

guest

guest pass

Any loc options following will be interpreted as providing a guest OS boot image to the guest system. For example, if you use this option, you can have the bootloader pre-load a Linux kernel image to RAM at the location specified by loc, then point the qvm process instance to this location rather than to a file to start its guest.

initrd

initrd pass

Any loc options following will be interpreted as providing an initial RAM disk (initrd) image to the Linux guest system.

raw

Recognized, but not relevant for pass-through.

For general information about pass-through devices, see Pass-through devices in the Understanding QNX Virtual Environments chapter.

Note:

On ARM platforms, devices that are clock-dependent (e.g., eMMC devices) can't be passed through (see Passing through clock-dependent devices in the same chapter).

DMA status of pass-through devices

When you are implementing a QNX Hypervisor system, you should specify the DMA status of any pass-through device you add to your system; that is, include either d (DMA device) or n (not DMA device) in the access type following the length argument. For example, this configuration:

pass loc mem:0x2000,0x1000,rn=0x1000

specifies that the device is not a DMA device.

If you're using smmuman for DMA configuration checking (which is the default assumption), and you specify neither d nor n or you specify both for the access type, the qvm process instance assembling the VM prints a fatal error message and aborts without starting the VM.

CAUTION:

A DMA device may not be used by more than one OS, whether it's multiple guests or the host and a guest. For example, if you configure a VM to give a guest access to a DMA device that has been already allocated to another VM, when the newly configured VM attempts to access the device, this will either succeed or fail depending on whether an external manager (e.g., smmuman) is controlling access to the device. If the access fails, qvm aborts and hence, the guest terminates immediately. Therefore, it's important to manage system-wide contention for devices independently of each VM.

Examples:

Linux RAM disk image

The following provides an initrd RAM disk image to a Linux system from host system memory:

initrd pass loc 0xB1234000,0x4000000,rc=0xB7654000

The above creates a RAM disk of 0x4000000 bytes located in the host-physical memory at address 0xB1234000, accessible by the guest at address 0xB7654000 in guest-physical memory. This region is cacheable (c) and read-only (r).

Note:

It is your responsibility to locate the data in the guest system in accordance with the guest OS's rules. You may prefer to use the load option to pass a Linux initrd RAM disk to a guest. For example: initrd load ./myinitrd.gz (see load in this chapter).

Audio device

The following passes through the audio-related devices:

pass pci:0:14.0 pass pci:0:23.0

Since these are PCI devices, the configuration uses BDF (bus/device/function) numbers rather than memory addresses. The example assumes a board with the specified physical devices at the specified locations.

Graphics device

Assuming that you have a screen driver and the other components you need to run graphics on the board, you could do the following to pass through a graphics device:

Modify your VM configuration file to create a PCI pass-through device:
```
pass loc pci:0:2.0=pci:0:2.0 
vdev pci-dummy 
    clone pci:0:31.0
```
Start the guest with the new VM configuration.
From your desktop host, start Screen in the guest, using the -c option to point screen to the DRM manager:
```
screen -c /usr/lib/graphics/intel-drm/graphics.conf
```

In step 1 of this example we use the 0:2.0 BDF location, which Intel has designated for integrated graphics:

The value to the left of the equals sign passes the host BDF through to the guest.
The value to the right of the equals sign ensures that the device will appear at the specified location on the guest's PCI bus.

We specify the BDF value in both places to ensure that the qvm process instance presents the same location (0:2.0) to the guest, in case code in the guest assumes this is an Intel-designated BDF.

The pci-dummy vdev is not strictly required. However, the device driver usually uses this vdev to identify the display adapter, so using the pci-dummy vdev allows us to expose the correct vendor and device IDs without providing any further (and unneeded) functionality.

The example assumes a board with the specified physical devices at the specified locations.

See also loc above, and pci-dummy in the Virtual Device Reference chapter.

Page updated: March 05, 2025