Thursday, October 25, 2018

Using bhyve for FreeBSD Development

Note: This was originally published as an article in the July/August 2014 issue of the FreeBSD Journal.  Some details have changed since the time of writing, for example bhyve now supports AMD-v extensions on AMD CPUs.  However, I still use the network setup I describe today and several folks have asked for an example of NATed VMs on a laptop, so I've republished the article below.

One of the exciting new features in FreeBSD 10.0 is the bhyve hypervisor.  Hypervisors and virtual machines are used in a wide variety of applications.  This article will focus on using bhyve as a
tool for aiding development of FreeBSD itself.  Not all of the details covered are specific to FreeBSD development, however, and many may prove useful for other applications.

Note that the bhyve hypervisor is under constant development and some of the features described have been added since FreeBSD 10.0 was released.  Most of these features should be present in FreeBSD 10.1.

The Hypervisor

The bhyve hypervisor requires a 64-bit x86 processor with hardware support for virtualization.  This requirement allows for a simple, clean hypervisor implementation, but it does require a fairly recent
processor.  The current hypervisor requires an Intel processor, but there is an active development branch with support for AMD processors.

The hypervisor itself contains both user and kernel components.  The kernel driver is contained in the vmm.ko module and can be loaded either at boot from the boot loader or at runtime.  It must
be loaded before any guests can be created.  When a guest is created, the kernel driver creates a device file in /dev/vmm which is used by the user programs to interact with the guest.

The primary user component is the bhyve(8) program.  It constructs the emulated device tree in the guest and provides the implementation for most of the emulated devices.  It also calls the kernel driver to execute the guest.  Note that the guest always executes inside the driver itself, so guest execution time in the host is counted as system time in the bhyve process.

Currently, bhyve does not provide a system firmware interface to the guest (neither BIOS nor UEFI).  Instead, a user program running on the host is used to perform boot time operations including loading the guest operating system kernel into the guest's memory and setting the initial guest state so that the guest begins execution at the kernel's entry point.  For FreeBSD guests, the bhyveload(8) program can be used to load the kernel and prepare the guest for execution.  Support for some other operating systems is available via the grub2-bhyve program which is available via the sysutils/grub2-bhyve port or as a prebuilt package.

The bhyveload(8) program in FreeBSD 10.0 only supports 64-bit guests.  Support for 32-bit guests will be included in FreeBSD 10.1.

Network Setup

The network connections between the guests and the host can be configured in several different ways.  Two different setups are described below, but they are not the only possible configurations.

The only guest network driver currently supported by bhyve is the VirtIO network interface driver.  Each network interface exposed to the guest is associated with a tap(4) interface in the host.  The tap(4) driver allows a user program to inject packets into the network stack and accept packets from the network stack.  By design, each tap(4) interface will only pass traffic if it is opened by a user process and it is administratively enabled via ifconfig(8).  As a result, each tap(4) interface must be explicitly enabled each time a guest is booted.  This can be inconvenient for frequently
restarted guests.  The tap(4) driver can be changed to automatically enable an interface when it is opened by a user process by setting the sysctl to 1.

Bridged Configuration

One simple network setup bridges the guest network interfaces directly onto a network to which the host is connected.  On the host, a single if_bridge(4) interface is created.  The tap(4) interfaces for the guest are added to the bridge along with the network interface on the host that is attached to the desired network.  Example 1 connects a guest using tap0 to a LAN on the host's re0 interface:

Example 1: Manually Connecting a Guest to the Host's LAN

# ifconfig bridge0 create
# ifconfig bridge0 addm re0
# ifconfig bridge0 addm tap0
# ifconfig bridge0 up

The guest can then configure the VirtIO network interface bound to tap0 for the LAN on the host's re0 interface using DHCP or a static address.

The /etc/rc.d/bridge script allows bridges to be configured during boot automatically by variables in /etc/rc.conf.  The autobridge_interfaces variable lists the bridge interfaces to configure.  For each bridge interface, a autobridge_name variable lists other network interfaces that should be added as bridge members.  The list can include shell globs to match multiple interfaces.  Note that /etc/rc.d/bridge will not create the named bridge interfaces.  They should be created by listing them in the cloned_interfaces variable along with the desired tap(4) interfaces.  Example 2 lists the /etc/rc.conf settings to create three tap(4) interfaces bridged to a local LAN on the host's re0 interface.

Example 2: Bridged Configuration


autobridge_bridge0="re0 tap*"
cloned_interfaces="bridge0 tap0 tap1 tap2"

Private Network with NAT

A more complex network setup creates a private network on the host for the guests and uses network address translation (NAT) to provide limited access from guests to other networks.  This may be a more appropriate setup when the host is mobile or connects to untrusted networks.

This setup also uses an if_bridge(4) interface, but only the tap(4) interfaces used by guests are added as members to the bridge.  The bridge members are assigned addresses from a private subnet.  The
bridge interface is assigned an address from the private subnet as well to connect the bridge to the host's network stack.  This allows the guests and host to communicate over the private subnet used by the bridge.

The host acts as a router for the guests to permit guest access to remote systems.  IP forwarding is enabled on the host and guest connections are translated via natd(8).  The guests use the host's address in the private subnet as their default route.

Example 3 lists the/etc/rc.conf settings to create three tap(4) interfaces and a bridge interface using the subnet.  It also translates network connections over the the host's wlan0 interface using natd(8).

Example 3: Private Network Configuration


cloned_interfaces="bridge0 tap0 tap1 tap2"

Using dnsmasq with a Private Network

The private network from the previous example works well, but it is a bit tedious to work with.  Guests must statically configure their network interfaces, and network connections between guests and the host must use hardcoded IP addresses.  The dnsmasq utility can alleviate much of the tedium.  It can be installed via the dns/dnsmasq port or as a prebuilt package.

The dnsmasq daemon provides a DNS forwarding server as well as a DHCP server.  It can serve local DNS requests to map the hostnames of its DHCP clients to the addresses it assigns to the clients.  For the private network setup, this means that each guest can use DHCP to configure its network interface.  In addition, all of the guests and the host can resolve each guest's hostname.

The dnsmasq daemon is configured by settings in the /usr/local/etc/dnsmasq.conf configuration file. A sample configuration file is installed by the port by default.  The configuration file suggests enabling the domain-needed and bogus-priv settings for the DNS server to avoid sending useless DNS requests to upstream DNS servers.  To enable the DHCP server, interface must be set to the network interface on the host where the server should run, and dhcp-range must be set to configure the range of IP addresses that can be assigned to guests.

Example 4 instructs the dnsmasq daemon to run a DHCP server on the bridge0 interface and assign a subset of the subnet to guests.

Example 4: Enabling dnsmasq's DNS and DHCP Servers



In addition to providing local DNS names for DHCP clients, dnsmasq also provides DNS names for any entries in /etc/hosts on the host.  An entry to /etc/hosts that maps the IP address assigned to bridge0 to a hostname (e.g. "host") will allow guests to use that hostname to contact the host.

The last thing remaining is to configure the host machine to use dnsmasq's DNS server.  Allowing the host to use dnsmasq's DNS server allows the host to resolve the name of each guest.  The dnsmasq daemon can use resolvconf(8) to seamlessly handle updates to the host's DNS configuration provided
by DHCP or VPN clients.  This is implemented by resolvconf(8) updating two configuration files that are read by dnsmasq each time the host's DNS configuration changes.  Finally, the host should always use dnsmasq's DNS server and rely on it to forward requests to other upstream DNS servers.  Enabling all this requires changes to both dnsmasq's configuration file and /etc/resolvconf.conf.  More details about configuring resolvconf(8) can be found in resolvconf.conf(5).  Example 5 gives the changes to both files to use dnsmasq as the host's name resolver.

Example 5: Use dnsmasq as the Host's Resolver





Running Guests via

Executing a guest requires several steps.  First, any state from a previous guest using the same name must be cleared before a new guest can begin.  This is done by passing the --destroy flag to
bhyvectl(8).  Second, the guest must be created and the guest's kernel must be loaded into its address space by bhyveload(8) or grub2-bhyve.  Finally, the bhyve(8) program is used to create virtual devices and provide runtime support for guest execution.  Doing this all by hand for each guest invocation can be a bit tedious, so FreeBSD ships with a wrapper script for FreeBSD guests:

The script manages a simple FreeBSD guest.  It performs the three steps above in a loop so that the guest restarts after a reboot similar to real hardware.  It provides a fixed set of virtual devices
to the guest including a network interface backed by a tap(4) interface, a local disk backed by a disk image, and an optional second disk backed by an install image.  To make guest installations easier, checks the provided disk image for a valid boot sector.  If none is found, it instructs bhyveload(8) to boot from the install image, otherwise it boots from the disk image.  In FreeBSD 10.1 and later, will terminate its loop when the guest requests soft off via ACPI.

The simplest way to bootstrap a new FreeBSD guest is to install the guest from an install ISO image.  For a FreeBSD guest running 9.2 or later, the standard install process can be used by using the normal
install ISO as the optional install image passed to  FreeBSD 8.4 also works as a bhyve guest.  However, its installer does not fully support VirtIO block devices, so the initial install must be
performed manually using steps similar to those from the RootOnZFS guide.  Example 6 creates a 64-bit guest named "vm0" and boots the install CD for FreeBSD 10.0-RELEASE.  Once the guest has been installed, the -I argument can be dropped to boot the guest from the disk image.

Example 6: Creating a FreeBSD/amd64 10.0 Guest

# mkdir vm0
# truncate -s 8g vm0/disk.img
# sh /usr/share/examples/bhyve/ -t tap0 -d vm0/disk.img \
  -I FreeBSD-10.0-RELEASE-amd64-disc1.iso vm0

The script runs bhyve(8) synchronously and uses its standard file descriptors as the backend of the first serial port assigned to the guest.  This serial port is used as the system console device for
FreeBSD guests.  The simplest way to run a guest in the background using is to use a tool such as screen or tmux.

FreeBSD 10.1 and later treat the SIGTERM signal sent to bhyve(8) as a virtual power button.  If the guest supports ACPI, then sending SIGTERM interrupts the guest to request a clean shutdown.  The guest should then initiate an ACPI soft-off which will terminate the loop.  If the guest does not respond to SIGTERM, the guest can still be forcefully terminated from the host via SIGKILL.  If the guest does not support ACPI, then SIGTERM will immediately terminate the guest.

The script accepts several different arguments to control the behavior of bhyve(8) and bhyveload(8), but these arguments only permit enabling a subset of the features supported by these programs.  To control all available features or use alternate virtual device configurations (e.g. multiple virtual drives or network interfaces), either invoke bhyveload(8) and bhyve(8) manually or use as the basis of a custom script.

Configuring Guests

FreeBSD guests do not require extensive configuration settings to run, and most settings can be set by the system installer.  However, there are a few conventions and additional settings which can be useful.

Out of the box, FreeBSD releases prior to 9.3 and 10.1 expect to use a video console and keyboard as the system console.  As such, they do not enable a login prompt on the serial console.  A login prompt
should be enabled on the serial console by editing /etc/ttys and marking the ttyu0 terminal
"on".  Note that this can be done from the host after the install has completed by mounting the disk image on the host using mdconfig(8).  (Note: Be sure the guest is no longer accessing the disk image
before mounting its filesystem on the host to avoid data corruption.)

If a guest requires network access, it will require configuration similar to that of a normal host.  This includes configuring the guest's network interface (vtnet0) and assigning a hostname.  A useful convention is to re-use the name of the guest ("vm0" in Example 6) as the hostname.  The sendmail(8) daemon may hang attempting to resolve the guest's hostname during boot.  This can be worked around by completely disabling sendmail(8) in the guest.  Finally, most guests with network access will want to enable remote logins via sshd(8).

Example 7 lists the /etc/rc.conf file for a simple FreeBSD guest.

Example 7: Simple FreeBSD Guest Configuration



Using a bhyve Guest as a Target

One way bhyve can be used while developing FreeBSD is to allow a host to debug a guest as if the guest were a remote target.  Specifically, a test kernel can be built on the host, booted inside of the guest, and debugged from the host using kgdb(1).

Once a guest is created and configured and a test kernel has been built on the host, the next step is to boot the guest with the test kernel.  The traditional method is to install the kernel into the
guest's filesystem either by exporting the build directory to the guest via NFS, copying the kernel into the guest over the network, or mounting the guest's filesystem on the host directly via mdconfig(8).
An alternate method is available via bhyveload(8) which is similar to booting a test machine over the network.

Using bhyveload(8)'s Host Filesystem

The bhyveload(8) program allows a directory on the host's filesystem to be exported to the loader environment.  This can be used to load a kernel and modules from a host filesystem rather than the guest's disk image.  The directory on the host's filesystem is passed to bhyveload(8) via the -h flag.  The bhyveload(8) program exports a host0: device to the loader environment.  The path passed to the host0: device in the loader environment is appended to the configured directory to generate a host pathname.  Note that the directory passed to bhyveload(8) must be an absolute pathname.

The script in FreeBSD 10.1 and later allow the directory to be set via the -H argument.  The script will convert a relative pathname to an absolute pathname before passing it to bhyveload(8).

Booting a test kernel from the host inside of the guest involves the following three steps:
  1. Install the kernel into the directory on the host by setting the DESTDIR variable to the directory when invoking make install or make installkernel.  A non-root user with write access to the directory can perform this step directly by setting the KMODOWN make variable to the invoking user.
  2. Pass the directory's path to bhyveload(8) either via the the -h flag to bhyveload(8) or the -H flag to
  3. Explicitly load the new kernel at the bhyveload(8) prompt via the loader path host0:/boot/kernel/kernel.

Example 8 installs a kernel with the kernel config GUEST into a host directory for the guest "vm0".  It uses's -H argument to specify the host directory passed to bhyveload(8).  It also shows the commands used at the loader prompt to boot the test kernel.

Example 8: Booting a Kernel from the Host

> cd ~/work/freebsd/head/sys/amd64/compile/GUEST
> make install DESTDIR=~/bhyve/vm0/host KMODOWN=john
> cd ~/bhyve
> sudo sh -t tap0 -d vm0/disk.img -H vm0/host vm0
OK unload
OK load host0:/boot/kernel/kernel
host0:/boot/kernel/kernel text=0x523888 data=0x79df8+0x10e2e8 syms=[0x8+0x9fb58+0x8+0xbaf41]
OK boot
Copyright (c) 1992-2014 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
        The Regents of the University of California. All rights reserved.
FreeBSD is a registered trademark of The FreeBSD Foundation.
FreeBSD 11.0-CURRENT #6 r261528M: Fri Feb  7 09:55:45 EST 2014 amd64

The guest can be configured to load a kernel from the host0: filesystem on the next boot using nextboot(8).  To boot the host0:/boot/kernel/kernel kernel, run nextboot -e bootfile=host0:/boot/kernel/kernel before
rebooting.  Note that this will not adjust the module path used to load kernel modules, so it only works with a monolothic kernel.

Using bhyve(8)'s Debug Port

The bhyve(8) hypervisor provides an optional debug port that can be used by the host to debug the guest's kernel using kgdb(1).  To use this feature, the guest kernel must include the bvmdebug device driver, the KDB kernel debugger, and the GDB debugger backend.  The debug port must also be
enabled by passing the -g flag to bhyve(8).  The flag requires an argument to specify the local TCP port on which bhyve(8) will listen for a connection from kgdb(1).  The script also accepts a -g flag which is passed through to bhyve(8).

When the guest boots, its kernel will detect the debug port as an available GDB backend automatically.  To connect kgdb(1) on the host to the guest, first enter the kernel debugger by setting the debug.kdb.enter system control node to a non-zero value.  At the debugger prompt, invoke the gdb command.  On the host, run kgdb(1) using the guest's kernel as the kernel image.  The target remote command can be used to connect to the TCP port passed to bhyve(8).  Once kgdb(1) attaches to the remote target, it can be used to debug the guest kernel.  Examples 9 and 10
demonstrate these steps using a guest kernel built on the host.

Example 9: Using kgdb(1) with bvmdebug: In the Guest

> sudo sh -t tap0 -d vm0/disk.img -H vm0/host -g 1234 vm0
OK load host0:/boot/kernel/kernel
host0:/boot/kernel/kernel text=0x523888 data=0x79df8+0x10e2e8 syms=[0x8+0x9fb58+0x8+0xbaf41]
OK boot
GDB: debug ports: bvm
GDB: current port: bvm
root@vm0:~ # sysctl debug.kdb.enter=1
debug.kdb.enter: 0KDB: enter: sysctl debug.kdb.enter
[ thread pid 693 tid 100058 ]
Stopped at      kdb_sysctl_enter+0x87:  movq    $0,kdb_why
db> gdb
(ctrl-c will return control to ddb)
Switching to gdb back-end
Waiting for connection from gdb
 -> 0
root@vm0:~ #

Example 10: Using kgdb(1) with bvmdebug: On the Host

> cd ~/work/freebsd/head/sys/amd64/compile/GUEST
> kgdb kernel.debug 
(kgdb) target remote localhost:1234
Remote debugging using localhost:1234
warning: Invalid remote reply: 
kdb_sysctl_enter (oidp=<value optimized out>, arg1=<value optimized out>, 
    arg2=1021, req=<value optimized out>) at ../../../kern/subr_kdb.c:446
446                     kdb_why = KDB_WHY_UNSET;
Current language:  auto; currently minimal
(kgdb) c

Using kgdb(1) with a Virtual Serial Port

A serial port can also be used to allow the host to debug the guest's kernel.  This can be done by loading the nmdm(4) driver on the host and using a nmdm(4) device for the serial port used
for debugging.

To avoid spewing garbage on the console, connect the nmdm(4) device to the second serial port.  This is enabled in the hypervisor by passing -l com2,/dev/nmdm0B to bhyve(8).  The guest must be
configured to use the second serial port for debugging by setting the kernel environment variable hint.uart.1.flags=0x80 from bhyveload(8).  The kgdb(1) debugger on the host connects to the guest by using target remote /dev/nmdm0A.


The bhyve hypervisor is a nice addition to a FreeBSD developer's toolbox.  Guests can be used both to develop new features and to test merges to stable branches.  The hypervisor has a wide variety of uses beyond developing FreeBSD as well.

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