freebsd-skq/share/man/man4/xnb.4

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.\" Authors: Alan Somers         (Spectra Logic Corporation)
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.\" $FreeBSD$
.\"
.Dd June 6, 2014
.Dt XNB 4
.Os
.Sh NAME
.Nm xnb
.Nd "Xen Paravirtualized Backend Ethernet Driver"
.Sh SYNOPSIS
To compile this driver into the kernel, place the following lines in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "options XENHVM"
.Cd "device xenpci"
.Ed
.Sh DESCRIPTION
The
.Nm
driver provides the back half of a paravirtualized
.Xr xen 4
network connection.
The netback and netfront drivers appear to their respective operating
systems as Ethernet devices linked by a crossover cable.
Typically,
.Nm
will run on Domain 0 and the netfront driver will run on a guest domain.
However, it is also possible to run
.Nm
on a guest domain.
It may be bridged or routed to provide the netfront
domain access to other guest domains or to a physical network.
.Pp
In most respects, the
.Nm
device appears to the OS as any other Ethernet device.
It can be configured at runtime entirely with
.Xr ifconfig 8 .
In particular, it supports MAC changing, arbitrary MTU sizes, checksum
offload for IP, UDP, and TCP for both receive and transmit, and TSO.
However, see
.Sx CAVEATS
before enabling txcsum, rxcsum, or tso.
.Sh SYSCTL VARIABLES
The following read-only variables are available via
.Xr sysctl 8 :
.Bl -tag -width indent
.It Va dev.xnb.%d.dump_rings
Displays information about the ring buffers used to pass requests between the
netfront and netback.
Mostly useful for debugging, but can also be used to
get traffic statistics.
.It Va dev.xnb.%d.unit_test_results
Runs a builtin suite of unit tests and displays the results.
Does not affect the operation of the driver in any way.
Note that the test suite simulates error conditions; this will result in
error messages being printed to the system log.
.El
.Sh SEE ALSO
.Xr arp 4 ,
.Xr netintro 4 ,
.Xr ng_ether 4 ,
.Xr xen 4 ,
.Xr ifconfig 8
.Sh HISTORY
The
.Nm
device driver first appeared in
.Fx 10.0 .
.Sh AUTHORS
.An -nosplit
The
.Nm
driver was written by
.An Alan Somers Aq Mt asomers@FreeBSD.org
and John Suykerbuyk.
.Sh CAVEATS
Packets sent through Xennet pass over shared memory, so the protocol includes
no form of link-layer checksum or CRC.
Furthermore, Xennet drivers always report to their hosts that they support
receive and transmit checksum offloading.
They "offload" the checksum calculation by simply skipping it.
That works fine for packets that are exchanged between two domains on the same
machine.
However, when a Xennet interface is bridged to a physical interface,
a correct checksum must be attached to any packets bound for that physical
interface.
Currently,
.Fx
lacks any mechanism for an Ethernet device to
inform the OS that newly received packets are valid even though their checksums
are not.
So if the netfront driver is configured to offload checksum calculations,
it will pass non-checksumed packets to
.Nm ,
which must then calculate the checksum in software before passing the packet
to the OS.
.Pp
For this reason, it is recommended that if
.Nm
is bridged to a physical interface, then transmit checksum offloading should be
disabled on the netfront.
The Xennet protocol does not have any mechanism for the netback to request
the netfront to do this; the operator must do it manually.
.Sh BUGS
The
.Nm
driver does not properly checksum UDP datagrams that span more than one
Ethernet frame.
Nor does it correctly checksum IPv6 packets.
To workaround that bug, disable transmit checksum offloading on the
netfront driver.