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IBM Virtual Management Channel Kernel Driver (IBMVMC)
Dave Engebretsen <>,
Adam Reznechek <>,
Steven Royer <>,
Bryant G. Ly <>,
Note: Knowledge of virtualization technology is required to understand
this document.
A good reference document would be:
The Virtual Management Channel (VMC) is a logical device which provides an
interface between the hypervisor and a management partition. This interface
is like a message passing interface. This management partition is intended
to provide an alternative to systems that use a Hardware Management
Console (HMC) - based system management.
The primary hardware management solution that is developed by IBM relies
on an appliance server named the Hardware Management Console (HMC),
packaged as an external tower or rack-mounted personal computer. In a
Power Systems environment, a single HMC can manage multiple POWER
processor-based systems.
Management Application
In the management partition, a management application exists which enables
a system administrator to configure the system’s partitioning
characteristics via a command line interface (CLI) or Representational
State Transfer Application (REST API's).
The management application runs on a Linux logical partition on a
POWER8 or newer processor-based server that is virtualized by PowerVM.
System configuration, maintenance, and control functions which
traditionally require an HMC can be implemented in the management
application using a combination of HMC to hypervisor interfaces and
existing operating system methods. This tool provides a subset of the
functions implemented by the HMC and enables basic partition configuration.
The set of HMC to hypervisor messages supported by the management
application component are passed to the hypervisor over a VMC interface,
which is defined below.
The VMC enables the management partition to provide basic partitioning
- Logical Partitioning Configuration
- Start, and stop actions for individual partitions
- Display of partition status
- Management of virtual Ethernet
- Management of virtual Storage
- Basic system management
Virtual Management Channel (VMC)
A logical device, called the Virtual Management Channel (VMC), is defined
for communicating between the management application and the hypervisor. It
basically creates the pipes that enable virtualization management
software. This device is presented to a designated management partition as
a virtual device.
This communication device uses Command/Response Queue (CRQ) and the
Remote Direct Memory Access (RDMA) interfaces. A three-way handshake is
defined that must take place to establish that both the hypervisor and
management partition sides of the channel are running prior to
sending/receiving any of the protocol messages.
This driver also utilizes Transport Event CRQs. CRQ messages are sent
when the hypervisor detects one of the peer partitions has abnormally
terminated, or one side has called H_FREE_CRQ to close their CRQ.
Two new classes of CRQ messages are introduced for the VMC device. VMC
Administrative messages are used for each partition using the VMC to
communicate capabilities to their partner. HMC Interface messages are used
for the actual flow of HMC messages between the management partition and
the hypervisor. As most HMC messages far exceed the size of a CRQ buffer,
a virtual DMA (RMDA) of the HMC message data is done prior to each HMC
Interface CRQ message. Only the management partition drives RDMA
operations; hypervisors never directly cause the movement of message data.
Remote Direct Memory Access is DMA transfer from the server to its
client or from the server to its partner partition. DMA refers
to both physical I/O to and from memory operations and to memory
to memory move operations.
Command/Response Queue a facility which is used to communicate
between partner partitions. Transport events which are signaled
from the hypervisor to partition are also reported in this queue.
Example Management Partition VMC Driver Interface
This section provides an example for the management application
implementation where a device driver is used to interface to the VMC
device. This driver consists of a new device, for example /dev/ibmvmc,
which provides interfaces to open, close, read, write, and perform
ioctl’s against the VMC device.
VMC Interface Initialization
The device driver is responsible for initializing the VMC when the driver
is loaded. It first creates and initializes the CRQ. Next, an exchange of
VMC capabilities is performed to indicate the code version and number of
resources available in both the management partition and the hypervisor.
Finally, the hypervisor requests that the management partition create an
initial pool of VMC buffers, one buffer for each possible HMC connection,
which will be used for management application session initialization.
Prior to completion of this initialization sequence, the device returns
EBUSY to open() calls. EIO is returned for all open() failures.
Management Partition Hypervisor
<---------------------------------------- |
ADD BUFFER RESPONSE | - Perform # HMCs Iterations
----------------------------------------> -
VMC Interface Open
After the basic VMC channel has been initialized, an HMC session level
connection can be established. The application layer performs an open() to
the VMC device and executes an ioctl() against it, indicating the HMC ID
(32 bytes of data) for this session. If the VMC device is in an invalid
state, EIO will be returned for the ioctl(). The device driver creates a
new HMC session value (ranging from 1 to 255) and HMC index value (starting
at index 0 and ranging to 254) for this HMC ID. The driver then does an
RDMA of the HMC ID to the hypervisor, and then sends an Interface Open
message to the hypervisor to establish the session over the VMC. After the
hypervisor receives this information, it sends Add Buffer messages to the
management partition to seed an initial pool of buffers for the new HMC
connection. Finally, the hypervisor sends an Interface Open Response
message, to indicate that it is ready for normal runtime messaging. The
following illustrates this VMC flow:
Management Partition Hypervisor
Interface Open
Add Buffer _
<---------------------------------------- |
Add Buffer Response | - Perform N Iterations
----------------------------------------> -
Interface Open Response
VMC Interface Runtime
During normal runtime, the management application and the hypervisor
exchange HMC messages via the Signal VMC message and RDMA operations. When
sending data to the hypervisor, the management application performs a
write() to the VMC device, and the driver RDMA’s the data to the hypervisor
and then sends a Signal Message. If a write() is attempted before VMC
device buffers have been made available by the hypervisor, or no buffers
are currently available, EBUSY is returned in response to the write(). A
write() will return EIO for all other errors, such as an invalid device
state. When the hypervisor sends a message to the management, the data is
put into a VMC buffer and an Signal Message is sent to the VMC driver in
the management partition. The driver RDMA’s the buffer into the partition
and passes the data up to the appropriate management application via a
read() to the VMC device. The read() request blocks if there is no buffer
available to read. The management application may use select() to wait for
the VMC device to become ready with data to read.
Management Partition Hypervisor
VMC Interface Close
HMC session level connections are closed by the management partition when
the application layer performs a close() against the device. This action
results in an Interface Close message flowing to the hypervisor, which
causes the session to be terminated. The device driver must free any
storage allocated for buffers for this HMC connection.
Management Partition Hypervisor
Additional Information
For more information on the documentation for CRQ Messages, VMC Messages,
HMC interface Buffers, and signal messages please refer to the Linux on
Power Architecture Platform Reference. Section F.