OpenCloudOS-Kernel/Documentation/driver-api/uio-howto.rst

=======================
The Userspace I/O HOWTO
=======================

:Author: Hans-Jürgen Koch Linux developer, Linutronix
:Date:   2006-12-11

About this document
===================

Translations
------------

If you know of any translations for this document, or you are interested
in translating it, please email me hjk@hansjkoch.de.

Preface
-------

For many types of devices, creating a Linux kernel driver is overkill.
All that is really needed is some way to handle an interrupt and provide
access to the memory space of the device. The logic of controlling the
device does not necessarily have to be within the kernel, as the device
does not need to take advantage of any of other resources that the
kernel provides. One such common class of devices that are like this are
for industrial I/O cards.

To address this situation, the userspace I/O system (UIO) was designed.
For typical industrial I/O cards, only a very small kernel module is
needed. The main part of the driver will run in user space. This
simplifies development and reduces the risk of serious bugs within a
kernel module.

Please note that UIO is not an universal driver interface. Devices that
are already handled well by other kernel subsystems (like networking or
serial or USB) are no candidates for an UIO driver. Hardware that is
ideally suited for an UIO driver fulfills all of the following:

-  The device has memory that can be mapped. The device can be
   controlled completely by writing to this memory.

-  The device usually generates interrupts.

-  The device does not fit into one of the standard kernel subsystems.

Acknowledgments
---------------

I'd like to thank Thomas Gleixner and Benedikt Spranger of Linutronix,
who have not only written most of the UIO code, but also helped greatly
writing this HOWTO by giving me all kinds of background information.

Feedback
--------

Find something wrong with this document? (Or perhaps something right?) I
would love to hear from you. Please email me at hjk@hansjkoch.de.

About UIO
=========

If you use UIO for your card's driver, here's what you get:

-  only one small kernel module to write and maintain.

-  develop the main part of your driver in user space, with all the
   tools and libraries you're used to.

-  bugs in your driver won't crash the kernel.

-  updates of your driver can take place without recompiling the kernel.

How UIO works
-------------

Each UIO device is accessed through a device file and several sysfs
attribute files. The device file will be called ``/dev/uio0`` for the
first device, and ``/dev/uio1``, ``/dev/uio2`` and so on for subsequent
devices.

``/dev/uioX`` is used to access the address space of the card. Just use
:c:func:`mmap()` to access registers or RAM locations of your card.

Interrupts are handled by reading from ``/dev/uioX``. A blocking
:c:func:`read()` from ``/dev/uioX`` will return as soon as an
interrupt occurs. You can also use :c:func:`select()` on
``/dev/uioX`` to wait for an interrupt. The integer value read from
``/dev/uioX`` represents the total interrupt count. You can use this
number to figure out if you missed some interrupts.

For some hardware that has more than one interrupt source internally,
but not separate IRQ mask and status registers, there might be
situations where userspace cannot determine what the interrupt source
was if the kernel handler disables them by writing to the chip's IRQ
register. In such a case, the kernel has to disable the IRQ completely
to leave the chip's register untouched. Now the userspace part can
determine the cause of the interrupt, but it cannot re-enable
interrupts. Another cornercase is chips where re-enabling interrupts is
a read-modify-write operation to a combined IRQ status/acknowledge
register. This would be racy if a new interrupt occurred simultaneously.

To address these problems, UIO also implements a write() function. It is
normally not used and can be ignored for hardware that has only a single
interrupt source or has separate IRQ mask and status registers. If you
need it, however, a write to ``/dev/uioX`` will call the
:c:func:`irqcontrol()` function implemented by the driver. You have
to write a 32-bit value that is usually either 0 or 1 to disable or
enable interrupts. If a driver does not implement
:c:func:`irqcontrol()`, :c:func:`write()` will return with
``-ENOSYS``.

To handle interrupts properly, your custom kernel module can provide its
own interrupt handler. It will automatically be called by the built-in
handler.

For cards that don't generate interrupts but need to be polled, there is
the possibility to set up a timer that triggers the interrupt handler at
configurable time intervals. This interrupt simulation is done by
calling :c:func:`uio_event_notify()` from the timer's event
handler.

Each driver provides attributes that are used to read or write
variables. These attributes are accessible through sysfs files. A custom
kernel driver module can add its own attributes to the device owned by
the uio driver, but not added to the UIO device itself at this time.
This might change in the future if it would be found to be useful.

The following standard attributes are provided by the UIO framework:

-  ``name``: The name of your device. It is recommended to use the name
   of your kernel module for this.

-  ``version``: A version string defined by your driver. This allows the
   user space part of your driver to deal with different versions of the
   kernel module.

-  ``event``: The total number of interrupts handled by the driver since
   the last time the device node was read.

These attributes appear under the ``/sys/class/uio/uioX`` directory.
Please note that this directory might be a symlink, and not a real
directory. Any userspace code that accesses it must be able to handle
this.

Each UIO device can make one or more memory regions available for memory
mapping. This is necessary because some industrial I/O cards require
access to more than one PCI memory region in a driver.

Each mapping has its own directory in sysfs, the first mapping appears
as ``/sys/class/uio/uioX/maps/map0/``. Subsequent mappings create
directories ``map1/``, ``map2/``, and so on. These directories will only
appear if the size of the mapping is not 0.

Each ``mapX/`` directory contains four read-only files that show
attributes of the memory:

-  ``name``: A string identifier for this mapping. This is optional, the
   string can be empty. Drivers can set this to make it easier for
   userspace to find the correct mapping.

-  ``addr``: The address of memory that can be mapped.

-  ``size``: The size, in bytes, of the memory pointed to by addr.

-  ``offset``: The offset, in bytes, that has to be added to the pointer
   returned by :c:func:`mmap()` to get to the actual device memory.
   This is important if the device's memory is not page aligned.
   Remember that pointers returned by :c:func:`mmap()` are always
   page aligned, so it is good style to always add this offset.

From userspace, the different mappings are distinguished by adjusting
the ``offset`` parameter of the :c:func:`mmap()` call. To map the
memory of mapping N, you have to use N times the page size as your
offset::

    offset = N * getpagesize();

Sometimes there is hardware with memory-like regions that can not be
mapped with the technique described here, but there are still ways to
access them from userspace. The most common example are x86 ioports. On
x86 systems, userspace can access these ioports using
:c:func:`ioperm()`, :c:func:`iopl()`, :c:func:`inb()`,
:c:func:`outb()`, and similar functions.

Since these ioport regions can not be mapped, they will not appear under
``/sys/class/uio/uioX/maps/`` like the normal memory described above.
Without information about the port regions a hardware has to offer, it
becomes difficult for the userspace part of the driver to find out which
ports belong to which UIO device.

To address this situation, the new directory
``/sys/class/uio/uioX/portio/`` was added. It only exists if the driver
wants to pass information about one or more port regions to userspace.
If that is the case, subdirectories named ``port0``, ``port1``, and so
on, will appear underneath ``/sys/class/uio/uioX/portio/``.

Each ``portX/`` directory contains four read-only files that show name,
start, size, and type of the port region:

-  ``name``: A string identifier for this port region. The string is
   optional and can be empty. Drivers can set it to make it easier for
   userspace to find a certain port region.

-  ``start``: The first port of this region.

-  ``size``: The number of ports in this region.

-  ``porttype``: A string describing the type of port.

Writing your own kernel module
==============================

Please have a look at ``uio_cif.c`` as an example. The following
paragraphs explain the different sections of this file.

struct uio_info
---------------

This structure tells the framework the details of your driver, Some of
the members are required, others are optional.

-  ``const char *name``: Required. The name of your driver as it will
   appear in sysfs. I recommend using the name of your module for this.

-  ``const char *version``: Required. This string appears in
   ``/sys/class/uio/uioX/version``.

-  ``struct uio_mem mem[ MAX_UIO_MAPS ]``: Required if you have memory
   that can be mapped with :c:func:`mmap()`. For each mapping you
   need to fill one of the ``uio_mem`` structures. See the description
   below for details.

-  ``struct uio_port port[ MAX_UIO_PORTS_REGIONS ]``: Required if you
   want to pass information about ioports to userspace. For each port
   region you need to fill one of the ``uio_port`` structures. See the
   description below for details.

-  ``long irq``: Required. If your hardware generates an interrupt, it's
   your modules task to determine the irq number during initialization.
   If you don't have a hardware generated interrupt but want to trigger
   the interrupt handler in some other way, set ``irq`` to
   ``UIO_IRQ_CUSTOM``. If you had no interrupt at all, you could set
   ``irq`` to ``UIO_IRQ_NONE``, though this rarely makes sense.

-  ``unsigned long irq_flags``: Required if you've set ``irq`` to a
   hardware interrupt number. The flags given here will be used in the
   call to :c:func:`request_irq()`.

-  ``int (*mmap)(struct uio_info *info, struct vm_area_struct *vma)``:
   Optional. If you need a special :c:func:`mmap()`
   function, you can set it here. If this pointer is not NULL, your
   :c:func:`mmap()` will be called instead of the built-in one.

-  ``int (*open)(struct uio_info *info, struct inode *inode)``:
   Optional. You might want to have your own :c:func:`open()`,
   e.g. to enable interrupts only when your device is actually used.

-  ``int (*release)(struct uio_info *info, struct inode *inode)``:
   Optional. If you define your own :c:func:`open()`, you will
   probably also want a custom :c:func:`release()` function.

-  ``int (*irqcontrol)(struct uio_info *info, s32 irq_on)``:
   Optional. If you need to be able to enable or disable interrupts
   from userspace by writing to ``/dev/uioX``, you can implement this
   function. The parameter ``irq_on`` will be 0 to disable interrupts
   and 1 to enable them.

Usually, your device will have one or more memory regions that can be
mapped to user space. For each region, you have to set up a
``struct uio_mem`` in the ``mem[]`` array. Here's a description of the
fields of ``struct uio_mem``:

-  ``const char *name``: Optional. Set this to help identify the memory
   region, it will show up in the corresponding sysfs node.

-  ``int memtype``: Required if the mapping is used. Set this to
   ``UIO_MEM_PHYS`` if you you have physical memory on your card to be
   mapped. Use ``UIO_MEM_LOGICAL`` for logical memory (e.g. allocated
   with :c:func:`kmalloc()`). There's also ``UIO_MEM_VIRTUAL`` for
   virtual memory.

-  ``phys_addr_t addr``: Required if the mapping is used. Fill in the
   address of your memory block. This address is the one that appears in
   sysfs.

-  ``resource_size_t size``: Fill in the size of the memory block that
   ``addr`` points to. If ``size`` is zero, the mapping is considered
   unused. Note that you *must* initialize ``size`` with zero for all
   unused mappings.

-  ``void *internal_addr``: If you have to access this memory region
   from within your kernel module, you will want to map it internally by
   using something like :c:func:`ioremap()`. Addresses returned by
   this function cannot be mapped to user space, so you must not store
   it in ``addr``. Use ``internal_addr`` instead to remember such an
   address.

Please do not touch the ``map`` element of ``struct uio_mem``! It is
used by the UIO framework to set up sysfs files for this mapping. Simply
leave it alone.

Sometimes, your device can have one or more port regions which can not
be mapped to userspace. But if there are other possibilities for
userspace to access these ports, it makes sense to make information
about the ports available in sysfs. For each region, you have to set up
a ``struct uio_port`` in the ``port[]`` array. Here's a description of
the fields of ``struct uio_port``:

-  ``char *porttype``: Required. Set this to one of the predefined
   constants. Use ``UIO_PORT_X86`` for the ioports found in x86
   architectures.

-  ``unsigned long start``: Required if the port region is used. Fill in
   the number of the first port of this region.

-  ``unsigned long size``: Fill in the number of ports in this region.
   If ``size`` is zero, the region is considered unused. Note that you
   *must* initialize ``size`` with zero for all unused regions.

Please do not touch the ``portio`` element of ``struct uio_port``! It is
used internally by the UIO framework to set up sysfs files for this
region. Simply leave it alone.

Adding an interrupt handler
---------------------------

What you need to do in your interrupt handler depends on your hardware
and on how you want to handle it. You should try to keep the amount of
code in your kernel interrupt handler low. If your hardware requires no
action that you *have* to perform after each interrupt, then your
handler can be empty.

If, on the other hand, your hardware *needs* some action to be performed
after each interrupt, then you *must* do it in your kernel module. Note
that you cannot rely on the userspace part of your driver. Your
userspace program can terminate at any time, possibly leaving your
hardware in a state where proper interrupt handling is still required.

There might also be applications where you want to read data from your
hardware at each interrupt and buffer it in a piece of kernel memory
you've allocated for that purpose. With this technique you could avoid
loss of data if your userspace program misses an interrupt.

A note on shared interrupts: Your driver should support interrupt
sharing whenever this is possible. It is possible if and only if your
driver can detect whether your hardware has triggered the interrupt or
not. This is usually done by looking at an interrupt status register. If
your driver sees that the IRQ bit is actually set, it will perform its
actions, and the handler returns IRQ_HANDLED. If the driver detects
that it was not your hardware that caused the interrupt, it will do
nothing and return IRQ_NONE, allowing the kernel to call the next
possible interrupt handler.

If you decide not to support shared interrupts, your card won't work in
computers with no free interrupts. As this frequently happens on the PC
platform, you can save yourself a lot of trouble by supporting interrupt
sharing.

Using uio_pdrv for platform devices
-----------------------------------

In many cases, UIO drivers for platform devices can be handled in a
generic way. In the same place where you define your
``struct platform_device``, you simply also implement your interrupt
handler and fill your ``struct uio_info``. A pointer to this
``struct uio_info`` is then used as ``platform_data`` for your platform
device.

You also need to set up an array of ``struct resource`` containing
addresses and sizes of your memory mappings. This information is passed
to the driver using the ``.resource`` and ``.num_resources`` elements of
``struct platform_device``.

You now have to set the ``.name`` element of ``struct platform_device``
to ``"uio_pdrv"`` to use the generic UIO platform device driver. This
driver will fill the ``mem[]`` array according to the resources given,
and register the device.

The advantage of this approach is that you only have to edit a file you
need to edit anyway. You do not have to create an extra driver.

Using uio_pdrv_genirq for platform devices
------------------------------------------

Especially in embedded devices, you frequently find chips where the irq
pin is tied to its own dedicated interrupt line. In such cases, where
you can be really sure the interrupt is not shared, we can take the
concept of ``uio_pdrv`` one step further and use a generic interrupt
handler. That's what ``uio_pdrv_genirq`` does.

The setup for this driver is the same as described above for
``uio_pdrv``, except that you do not implement an interrupt handler. The
``.handler`` element of ``struct uio_info`` must remain ``NULL``. The
``.irq_flags`` element must not contain ``IRQF_SHARED``.

You will set the ``.name`` element of ``struct platform_device`` to
``"uio_pdrv_genirq"`` to use this driver.

The generic interrupt handler of ``uio_pdrv_genirq`` will simply disable
the interrupt line using :c:func:`disable_irq_nosync()`. After
doing its work, userspace can reenable the interrupt by writing
0x00000001 to the UIO device file. The driver already implements an
:c:func:`irq_control()` to make this possible, you must not
implement your own.

Using ``uio_pdrv_genirq`` not only saves a few lines of interrupt
handler code. You also do not need to know anything about the chip's
internal registers to create the kernel part of the driver. All you need
to know is the irq number of the pin the chip is connected to.

Using uio_dmem_genirq for platform devices
------------------------------------------

In addition to statically allocated memory ranges, they may also be a
desire to use dynamically allocated regions in a user space driver. In
particular, being able to access memory made available through the
dma-mapping API, may be particularly useful. The ``uio_dmem_genirq``
driver provides a way to accomplish this.

This driver is used in a similar manner to the ``"uio_pdrv_genirq"``
driver with respect to interrupt configuration and handling.

Set the ``.name`` element of ``struct platform_device`` to
``"uio_dmem_genirq"`` to use this driver.

When using this driver, fill in the ``.platform_data`` element of
``struct platform_device``, which is of type
``struct uio_dmem_genirq_pdata`` and which contains the following
elements:

-  ``struct uio_info uioinfo``: The same structure used as the
   ``uio_pdrv_genirq`` platform data

-  ``unsigned int *dynamic_region_sizes``: Pointer to list of sizes of
   dynamic memory regions to be mapped into user space.

-  ``unsigned int num_dynamic_regions``: Number of elements in
   ``dynamic_region_sizes`` array.

The dynamic regions defined in the platform data will be appended to the
`` mem[] `` array after the platform device resources, which implies
that the total number of static and dynamic memory regions cannot exceed
``MAX_UIO_MAPS``.

The dynamic memory regions will be allocated when the UIO device file,
``/dev/uioX`` is opened. Similar to static memory resources, the memory
region information for dynamic regions is then visible via sysfs at
``/sys/class/uio/uioX/maps/mapY/*``. The dynamic memory regions will be
freed when the UIO device file is closed. When no processes are holding
the device file open, the address returned to userspace is ~0.

Writing a driver in userspace
=============================

Once you have a working kernel module for your hardware, you can write
the userspace part of your driver. You don't need any special libraries,
your driver can be written in any reasonable language, you can use
floating point numbers and so on. In short, you can use all the tools
and libraries you'd normally use for writing a userspace application.

Getting information about your UIO device
-----------------------------------------

Information about all UIO devices is available in sysfs. The first thing
you should do in your driver is check ``name`` and ``version`` to make
sure your talking to the right device and that its kernel driver has the
version you expect.

You should also make sure that the memory mapping you need exists and
has the size you expect.

There is a tool called ``lsuio`` that lists UIO devices and their
attributes. It is available here:

http://www.osadl.org/projects/downloads/UIO/user/

With ``lsuio`` you can quickly check if your kernel module is loaded and
which attributes it exports. Have a look at the manpage for details.

The source code of ``lsuio`` can serve as an example for getting
information about an UIO device. The file ``uio_helper.c`` contains a
lot of functions you could use in your userspace driver code.

mmap() device memory
--------------------

After you made sure you've got the right device with the memory mappings
you need, all you have to do is to call :c:func:`mmap()` to map the
device's memory to userspace.

The parameter ``offset`` of the :c:func:`mmap()` call has a special
meaning for UIO devices: It is used to select which mapping of your
device you want to map. To map the memory of mapping N, you have to use
N times the page size as your offset::

        offset = N * getpagesize();

N starts from zero, so if you've got only one memory range to map, set
``offset = 0``. A drawback of this technique is that memory is always
mapped beginning with its start address.

Waiting for interrupts
----------------------

After you successfully mapped your devices memory, you can access it
like an ordinary array. Usually, you will perform some initialization.
After that, your hardware starts working and will generate an interrupt
as soon as it's finished, has some data available, or needs your
attention because an error occurred.

``/dev/uioX`` is a read-only file. A :c:func:`read()` will always
block until an interrupt occurs. There is only one legal value for the
``count`` parameter of :c:func:`read()`, and that is the size of a
signed 32 bit integer (4). Any other value for ``count`` causes
:c:func:`read()` to fail. The signed 32 bit integer read is the
interrupt count of your device. If the value is one more than the value
you read the last time, everything is OK. If the difference is greater
than one, you missed interrupts.

You can also use :c:func:`select()` on ``/dev/uioX``.

Generic PCI UIO driver
======================

The generic driver is a kernel module named uio_pci_generic. It can
work with any device compliant to PCI 2.3 (circa 2002) and any compliant
PCI Express device. Using this, you only need to write the userspace
driver, removing the need to write a hardware-specific kernel module.

Making the driver recognize the device
--------------------------------------

Since the driver does not declare any device ids, it will not get loaded
automatically and will not automatically bind to any devices, you must
load it and allocate id to the driver yourself. For example::

     modprobe uio_pci_generic
     echo "8086 10f5" > /sys/bus/pci/drivers/uio_pci_generic/new_id

If there already is a hardware specific kernel driver for your device,
the generic driver still won't bind to it, in this case if you want to
use the generic driver (why would you?) you'll have to manually unbind
the hardware specific driver and bind the generic driver, like this::

        echo -n 0000:00:19.0 > /sys/bus/pci/drivers/e1000e/unbind
        echo -n 0000:00:19.0 > /sys/bus/pci/drivers/uio_pci_generic/bind

You can verify that the device has been bound to the driver by looking
for it in sysfs, for example like the following::

        ls -l /sys/bus/pci/devices/0000:00:19.0/driver

Which if successful should print::

      .../0000:00:19.0/driver -> ../../../bus/pci/drivers/uio_pci_generic

Note that the generic driver will not bind to old PCI 2.2 devices. If
binding the device failed, run the following command::

      dmesg

and look in the output for failure reasons.

Things to know about uio_pci_generic
------------------------------------

Interrupts are handled using the Interrupt Disable bit in the PCI
command register and Interrupt Status bit in the PCI status register.
All devices compliant to PCI 2.3 (circa 2002) and all compliant PCI
Express devices should support these bits. uio_pci_generic detects
this support, and won't bind to devices which do not support the
Interrupt Disable Bit in the command register.

On each interrupt, uio_pci_generic sets the Interrupt Disable bit.
This prevents the device from generating further interrupts until the
bit is cleared. The userspace driver should clear this bit before
blocking and waiting for more interrupts.

Writing userspace driver using uio_pci_generic
------------------------------------------------

Userspace driver can use pci sysfs interface, or the libpci library that
wraps it, to talk to the device and to re-enable interrupts by writing
to the command register.

Example code using uio_pci_generic
----------------------------------

Here is some sample userspace driver code using uio_pci_generic::

    #include <stdlib.h>
    #include <stdio.h>
    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>
    #include <errno.h>

    int main()
    {
        int uiofd;
        int configfd;
        int err;
        int i;
        unsigned icount;
        unsigned char command_high;

        uiofd = open("/dev/uio0", O_RDONLY);
        if (uiofd < 0) {
            perror("uio open:");
            return errno;
        }
        configfd = open("/sys/class/uio/uio0/device/config", O_RDWR);
        if (configfd < 0) {
            perror("config open:");
            return errno;
        }

        /* Read and cache command value */
        err = pread(configfd, &command_high, 1, 5);
        if (err != 1) {
            perror("command config read:");
            return errno;
        }
        command_high &= ~0x4;

        for(i = 0;; ++i) {
            /* Print out a message, for debugging. */
            if (i == 0)
                fprintf(stderr, "Started uio test driver.\n");
            else
                fprintf(stderr, "Interrupts: %d\n", icount);

            /****************************************/
            /* Here we got an interrupt from the
               device. Do something to it. */
            /****************************************/

            /* Re-enable interrupts. */
            err = pwrite(configfd, &command_high, 1, 5);
            if (err != 1) {
                perror("config write:");
                break;
            }

            /* Wait for next interrupt. */
            err = read(uiofd, &icount, 4);
            if (err != 4) {
                perror("uio read:");
                break;
            }

        }
        return errno;
    }

Generic Hyper-V UIO driver
==========================

The generic driver is a kernel module named uio_hv_generic. It
supports devices on the Hyper-V VMBus similar to uio_pci_generic on
PCI bus.

Making the driver recognize the device
--------------------------------------

Since the driver does not declare any device GUID's, it will not get
loaded automatically and will not automatically bind to any devices, you
must load it and allocate id to the driver yourself. For example, to use
the network device class GUID::

     modprobe uio_hv_generic
     echo "f8615163-df3e-46c5-913f-f2d2f965ed0e" > /sys/bus/vmbus/drivers/uio_hv_generic/new_id

If there already is a hardware specific kernel driver for the device,
the generic driver still won't bind to it, in this case if you want to
use the generic driver for a userspace library you'll have to manually unbind
the hardware specific driver and bind the generic driver, using the device specific GUID
like this::

          echo -n ed963694-e847-4b2a-85af-bc9cfc11d6f3 > /sys/bus/vmbus/drivers/hv_netvsc/unbind
          echo -n ed963694-e847-4b2a-85af-bc9cfc11d6f3 > /sys/bus/vmbus/drivers/uio_hv_generic/bind

You can verify that the device has been bound to the driver by looking
for it in sysfs, for example like the following::

        ls -l /sys/bus/vmbus/devices/ed963694-e847-4b2a-85af-bc9cfc11d6f3/driver

Which if successful should print::

      .../ed963694-e847-4b2a-85af-bc9cfc11d6f3/driver -> ../../../bus/vmbus/drivers/uio_hv_generic

Things to know about uio_hv_generic
-----------------------------------

On each interrupt, uio_hv_generic sets the Interrupt Disable bit. This
prevents the device from generating further interrupts until the bit is
cleared. The userspace driver should clear this bit before blocking and
waiting for more interrupts.

When host rescinds a device, the interrupt file descriptor is marked down
and any reads of the interrupt file descriptor will return -EIO. Similar
to a closed socket or disconnected serial device.

The vmbus device regions are mapped into uio device resources:
    0) Channel ring buffers: guest to host and host to guest
    1) Guest to host interrupt signalling pages
    2) Guest to host monitor page
    3) Network receive buffer region
    4) Network send buffer region

If a subchannel is created by a request to host, then the uio_hv_generic
device driver will create a sysfs binary file for the per-channel ring buffer.
For example:
	/sys/bus/vmbus/devices/3811fe4d-0fa0-4b62-981a-74fc1084c757/channels/21/ring

Further information
===================

-  `OSADL homepage. <http://www.osadl.org>`_

-  `Linutronix homepage. <http://www.linutronix.de>`_