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[=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[,{t|u}][=vector_number]
The guest_intr argument is associated with a host vector number, either vector_number if this value is specified, or the value specified by the hostvector option for the PIC identified by guest_intr.
When the host system delivers an interrupt on the host vector, guest_intr is delivered into the guest system.

The following attributes specify special behaviors:

t
The default method for handling interrupts is with an ISR (Interrupt Service Routine) rather than an IST (Interrupt Service Thread). Specify the intr t attribute to override the default method and to have your system use an IST (e.g., pass intr guest_intr,t).
Note:
Currently, ISR delivery is supported only if the targeted guest's interrupt controller is the gic vdev used on ARM boards. x86 boards will use IST delivery only, so the t attribute has no effect on these boards.
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 location_spec[,length][,(+|-|r|w|x|c|e|m|d|n|s|t)*]|[=host_address]
The guest system is allowed pass-through access to the host-physical memory address. The loc option supports the following arguments:
location_spec:
The device type and information required to locate it. The table below presents supported device types, and the location information each type requires:
Device Location
io:

Location in the x86 I/O space. This location is a port number. For example:

io:4

specifies an x86 I/O device on port 4.

The host_address argument is not used.
mem:

Guest-physical address of the pass-through device (the address in memory as seen by the guest). For example:

pass loc mem:0x2000,r=0x1000

specifies a read-only location at guest-physical address of 0x2000 and a host-physical address (address seen by the host) of 0x1000.

This is the default device type.
pci:

The BDF (bus/device/function) where the pass-through PCI device should appear in the guest. Here, host_address refers to the host device (vendor ID, device ID) being 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.

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.

The length argument and the flags are not used.

You can also specify PCI devices with just the Vendor ID and Device ID: pci:vendor_id/device_id; for example:

pass loc pci:0x8086/0x1234

See Example (graphics device) for some configuration examples.
length
The number of bytes allocated. The value specified for this argument must be a 32-bit value (i.e., not greater than 4294967295 or 0xFFFFFFFF).
access type
The access type specifies the types of access permitted to the guest. This argument must follow the length argument and precede the address, and may be a combination of the following:
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 are using the QHS, you must not also specify d.
d Device uses DMA. If you are using the QHS, you must specify either this argument or n.
n Device doesn't use DMA. If you are using the QHS, you must specify either this argument 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.
Note:

If a device uses DMA, you should use the d attribute, and specify at least the r and w access types. If smmuman isn't running, the qvm process instance for which the pass-through device is intended will refuse to include the device in its VM configuration, and proceed as specified by the safety option in the VM's configuration.

host_address

The physical address of the device on the system (i.e., as seen by the hypervisor host). For some location types (e.g., mem:), you can use location_spec to provide the guest with an address different from the host-physical address of the device on the system; the device is at location A, but the guest sees it at location B.

Note:
Not all device types (e.g., IO devices) can be relocated to different locations between the guest and the host.
CAUTION:
When the pass loc option is used to pass RAM through to a guest, 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 RAM may contain vestigial data from a previous guest system.
sched priority
Set the priority of the pulses the host system uses to inform the qvm processs 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 Neutrino 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 pass-through devices

When you are implementing a system with the QNX Hypervisor for Safety, you must specify the DMA status of every pass-through device you add to your system; that is, you must include either d (DMA device) or n (not DMA device) in the access type following the length argument. For example: 
pass loc mem:0x2000,0x1000,rn=0x1000
specifies that the device isn't a DMA device.

If you specify neither d nor n for the access type (or if you specify both), the qvm process instance assembling the VM will turn off the system's safety status indicator and respond as specified by the safety option in the VM's configuration.

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. The hypervisor responds based on the safety option setting. Therefore, it's important to manage system-wide contention for devices independently of each VM.
Warning:
For more information about what you may and may not do when configuring VMs in a QHS system, see the QNX Hypervisor for Safety 2.2 Safety Manual.

Example (Linux RAM disk image)

The following provides an initrd RAM disk image to the 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 domain. For example: initrd load ./myinitrd.gz (see load in this chapter).

Example (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.

Example (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:

  1. 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
  2. Start the guest with the new VM configuration.
  3. From your desktop host, start QNX 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 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 dummy vdev is not strictly required. However, the device driver usually uses this vdev to identify the display adapter, so using the 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 vdev pci-dummy in the Virtual Device Reference chapter.

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