Provided by: bit-babbler_0.8_amd64 bug


       bbvirt - hotplug BitBabbler devices into libvirt managed domains


       bbvirt action [options]

       bbvirt attach|detach device [options]

       bbvirt attach-all|detach-all [domain] [options]


       The  bbvirt  program  is  an  attempt  to  take  some of the pain out of what is currently
       required to distribute multiple USB devices between the host and guest  virtual  machines.
       While  there are several ways in which this may be configured and managed, at present none
       of them actually provide a complete and coherent solution on their own, all of  them  fall
       short of the mark in some significant and annoying way.  The aim here is to piece together
       enough of those hacks to actually get all of the functionality that we want now, until the
       libvirt native support for this improves enough to not need it anymore.

       At present this deals with libvirt managed QEMU/KVM virtual machines.

   What do we want?
       The  ideal  behaviour  here  is  pretty simple.  Given some arbitrary number of BitBabbler
       devices, we should be able to assign them to either the host machine, or  to  a  guest  VM
       running on it, and once we do that they should behave in the normal manner expected of any
       USB device.

       - If they are plugged in when the guest machine is started, they should be  seen  by  that
         machine as they would be by the host.

       - If they are plugged in after the machine is started, they should be hotplugged into that
         machine as they would be on the host.

       - If they are unplugged while the machine is running, they should be cleanly removed  from
         it, as they would be on the host.

   Why can't we have it?
       Right  now,  libvirt  gives  us two ways that we can assign USB devices from the host to a
       guest domain.

       - We can assign them by their USB vendor and product ID.  But that only works  when  there
         is  just  a  single device of that type in the host.  Which is pretty useless in most of
         the cases that we care about here, where the host and each of the guests are  likely  to
         have one or more BitBabbler devices of their own assigned to them.

       - We  can  assign them by their logical address on the USB bus.  But that isn't a constant
         that we can statically configure for the domain.  Every time a device is plugged in,  or
         replugged,  or  reset, or the host machine is rebooted, that address is likely to change
         since it is dynamically allocated when the device is enumerated on the bus.

       There is a third way, but it relies on bypassing the normal libvirt configuration to  make
       direct  use  of  the  QEMU  ability to assign a device by its physical address on the bus.
       Which is better, but still not a magic bullet since it relies on plugging exactly the same
       devices  into  exactly  the same ports every time (and on having those ports enumerated in
       the same way by the host on every reboot, which isn't guaranteed either).  It also  forces
       us  to  jump through other hoops, since we then need additional complication to manage the
       access permissions of the device manually outside of libvirt, but  still  in  coordination
       with it.

       The  even bigger failing, which all of those methods have in common, is they all depend on
       the device already being plugged in before the guest is started.  If it is inserted  after
       the  guest is started, or removed and replugged while the guest is running, or if the host
       bus or a hub bounces causing a reconnect, then the device will not be (re)attached to  the
       guest.   The only way to fix that if it happens is to manually reattach the device with an
       arcane incantation in XML (which relies on you knowing the new address of the device),  or
       to  completely  power  down  and  restart  the  guest.   Not the pinnacle of user-friendly
       operation that we are looking for here.

   What can we do about it?
       There was a patch submitted to libvirt some years back which would have allowed  a  device
       to  be specified by both its USB product ID and its serial number, but that got some push-
       back, and so far has still not been applied upstream.  That would have  gone  a  long  way
       toward  making  this  both easy and clean, leaving us only with the hotplug aspect to deal
       with.  We'll leave grumpy snark about that as an exercise for the reader ...

       Another alternative is we can delegate finding the device's logical address to  a  hotplug
       manager  like  udev(7).  This is attractive in the sense that we can know when the address
       of a device changes and what it changes to, but udev itself isn't  very  friendly  to  the
       idea  of  local  admin  customisation  (while it is possible to do, it seems to be getting
       increasingly strongly discouraged) and using it  still  requires  some  external  glue  to
       translate  its  events  into  something  that  libvirt  can  act on to configure the guest

       The bbvirt program provides that glue, and a  user  friendly  method  of  assigning  which
       devices should belong to which guest domains, and a front end that can be invoked manually
       or by other admin controlled tasks to quickly and easily add or remove BitBabbler  devices
       from any of the running guest machines.

       But the limitation this approach has, is that it can't easily know when a guest machine is
       started which should have devices that are already plugged in added to it.  In  theory  we
       could  add them to its persistent domain definition, but that has its own problems because
       we can only add devices by their ephemeral logical address, and we can't guarantee that we
       will  get  called  to  remove them from the domain again when that address becomes invalid
       (like if the host is suddenly powered off or it is otherwise not cleanly shut down), so we
       could  end up with many stale entries accumulating in the persistent domain configuration,
       which could later match some completely different device to what we had wanted attached to
       it.  Which means until that somehow gets fixed, it's only safe to add them to a live guest
       domain, so that they will always be removed again when it is  halted,  no  matter  how  it
       ended up getting halted.

       Clearly we've still got some way to go to get to our ideal here.

   What if we hit it with *two* hammers?
       There  appears  to  be  only  two  ways  that we can get notified of a guest machine being
       started at present.  One involves running yet  another  daemon  process,  which  would  do
       little  more than just sit around waiting for someone to start a guest so it could tell us
       about that.  But then we'd have yet  another  thing  to  configure,  yet  another  process
       running,  and  yet more problems with figuring out how to ensure we don't lose a race when
       the host is booted, between getting the initial set of device events, that  process  being
       ready and active, and any guests that will be autostarted at boot actually starting.

       The  other  way  is  to use a libvirt hook.  Which in turn has the problem of not actually
       allowing us to run any libvirt functions from it, which we need to do in order  to  attach
       the device to the host.  And which we can't guarantee that we can just install by default,
       because there can be only one such hook on the system, which the local admin  may  already
       be using ...

       There  is a third way, but that would involve requiring the local admin to start all guest
       machines through a wrapper of our own, instead of via whatever mechanism they already know
       and  use.   Which  doesn't scale to support other USB devices in the same situation, among
       the many ways that would be a horrible solution to inflict on people.

       But there is a loophole we can exploit.  We can use the libvirt qemu  hook  to  trigger  a
       change  event  for  udev,  which can in turn invoke bbvirt in much the same way that would
       happen if the device was really hotplugged, which gives us the extra layer of  indirection
       we  need  to  be  able to safely do that from the hook.  Rube Goldberg would be proud, and
       some of the pieces may require hand-assembly, but with all of this in place, we  can  have
       something resembling normal USB functionality in the guest machines.

       It's not pretty, but it will work with what we have to work with.

   Ok, just tell me where to hit it.
       To string this together, you'll need to ensure all of the following:

       - The  udev(7)  rules  from  the bit-babbler package are installed.  If you installed this
         from the Debian packages that should already be done.  If you didn't, you will  need  to
         install the rules that are found in debian/bit-babbler.udev from the source package to a
         suitable place on your system (probably /etc/udev/rules.d).

       - The bbvirt(1) script is installed in a place where the udev rules will find it.  If  you
         didn't install this from the Debian packages, and it isn't in /usr/bin, then you'll need
         to tweak the udev rules to suit.

       - The devices you wish to use in guest machines, and the machines you wish to use them in,
         are  specified  in  the  bbvirt  configuration  file.   The default location for that is
         /etc/bit-babbler/vm.conf.  If you wish to use a different file you will need to pass its
         location with the --config option in the udev rules, and update the hook script use that
         file too.  The details  of  what  you  can  put  in  that  file  are  described  in  the
         CONFIGURATION OPTIONS section below.

       - The  libvirt  hook  file  is  installed.  If all the above is done, then devices will be
         added to the running guest machines if they get plugged in while the guest  is  running.
         This  last  step  ensures  devices  which  are already plugged in will be added to newly
         started guests too (which includes guests that are started automatically when  the  host
         machine boots).

         Until there is some safe way we can install this without conflicting with or overwriting
         an existing hook, everyone will need to do this step manually.  If  you  have  installed
         the  Debian  packages,  then the example hook script that we've provided for this can be
         found in /usr/share/doc/bit-babbler/examples/qemu-hook.  If you didn't it can  be  found
         in libvirt/qemu-hook of the source package.

         You will need to install that file as /etc/libvirt/hooks/qemu, or merge its content with
         the existing qemu file there if you already have that hook set.  If that  file  did  not
         previously exist, you will need to restart libvirtd(8) to get it to begin using it.

       That should cover all of the needed automation, but you can also attach and detach devices
       manually at any time too.  The details of doing that will be described  in  the  following
       section.   Otherwise,  with all the above done, there is no other reason to need to invoke
       bbvirt directly.


       There are two primary modes of operation for bbvirt which  are  selected  by  the  initial
       action  option.   If  the  action to perform is attach or detach then only a single device
       will be acted upon, and which device that should be must be specified explicitly, even  if
       there  is only one device present on the host at the time.  When invoking bbvirt manually,
       the device may be specified by its serial number, its logical address on the bus  (in  the
       form busnum:devnum, given as decimal integers), or its physical address on the bus (in the
       form busnum-port[.port ...]).

       If the action to perform is attach-all or detach-all, then the device(s) to act  upon  are
       selected  by  domain  association  instead.  If a domain is explicitly specified, then all
       devices which are assigned to that guest domain in the configuration file  will  be  acted
       upon  in  the  same  way  as  if bbvirt was invoked for each of them individually with the
       attach or detach action.  If no domain is provided,  then  all  of  the  configured  guest
       domains will be acted upon in this way.

       The following additional options are available:

       -C, --config
              Specify  an  alternative  configuration file to import the device assignments from.
              If the path to the file is not provided explicitly, then it will be looked  for  in
              the /etc/bit-babbler directory (with a .conf suffix).

       -c, --connect=URI
              Specify  the  virsh(1)  connection URI to use.  This will override a DOMAIN_URI set
              for the domain in the configuration file.  If that is not set using either of these
              methods then the virsh default for the user running bbvirt will be used.

       -D, --domain=name
              Specify  the  libvirt  domain to act upon.  This may be used to override the device
              allocation from the configuration file when bbvirt is invoked manually, or  to  act
              on a device or domain that is not currently specified in the configuration file.

       -b, --busnum=num
              Specify  the  USB bus number that the device is attached to.  This option is mostly
              used to avoid bbvirt needing to look this up when it is already known (such as when
              it  is  called  from a udev rule).  There isn't usually much reason to pass this if
              invoking bbvirt manually, since you can just specify the device by its  logical  or
              physical address instead.

       -d, --devnum=num
              Specify the USB device number that the device is currently assigned.  Together with
              the bus number, this forms the logical address  of  the  device.   This  option  is
              mostly  used to avoid bbvirt needing to look this up when it is already known (such
              as when it is called from a udev rule).  There isn't usually much  reason  to  pass
              this  if  invoking  bbvirt  manually,  since you can just specify the device by its
              logical address instead.

       -n, --dry-run
              Don't attach or detach any devices, just show what would be attempted if this was a
              live  run.  This option implies a minimal level of --verbose, but the verbosity may
              be increased further by also passing that option explicitly.

       -v, --verbose
              Make more noise about what is really going on.  It may be passed multiple times  to
              increase the verbosity further.

       -?, --help
              Show a brief summary of the available options.


       The  bbvirt  configuration  file  contains  variable  assignments  using the bash(1) shell
       syntax.  It is sourced as a shell  snippet,  so  you  could  in  principle  construct  the
       configuration  for  each domain dynamically, but most typically a simple static assignment
       of devices to domains will suffice.  If you do elect to run code in it, you should be very
       defensive  about  namespacing  any  other variables you use, or any other side effects you
       might cause to happen.  Any number of guest domains may be configured in it.

       For each guest domain, three variables control the behaviour of bbvirt:

              This variable is optional if guestname and domain are the same.  It must be used if
              the  libvirt guest name contains any characters which would not be valid for use as
              a shell variable name (i.e. anything that is  not  ASCII  a-z,  A-Z,  0-9,  or  the
              underscore).  If set, it indicates that the corresponding DOMAIN_*_domain variables
              shown below are configuration for the libvirt guest domain guestname instead of one
              with the name domain.

              When  specifying  a domain option for bbvirt to act upon, you may use either of the
              domain or guestname identifiers interchangeably.

              This variable is optional, and  sets  the  virsh(1)  connection  URI  to  use  when
              attaching  or  detaching devices from the given domain.  If the --connect option is
              explicitly passed to bbvirt it will override what is set here.  If  the  connection
              URI  is  not  set using either of these methods then the virsh default for the user
              running bbvirt will be used (which would normally be root if run from udev).

       DOMAIN_RNG_domain=( device serial numbers ... )
              This variable is required if automatic  passthrough  of  devices  to  a  domain  is
              desired.   It  is  a  bash  array, populated with a space separated list of all the
              device serial numbers that you want assigned to domain.  It is  not  an  error  for
              devices  to  be listed here which are not currently plugged in.  It is important to
              ensure that devices are only assigned  to  one  domain  though,  and  that  devices
              assigned  to  guest  domains will not be used by a seedd(1) instance running on the
              host (which means the seedd configuration needs to be passed an  explicit  list  of
              the devices that it may use too).

              The  device serial number must always be used here.  You cannot specify a device by
              its logical or physical address on the bus (like you can in most other places where
              we take a device ID).


              The  default configuration file for assigning BitBabbler devices to libvirt managed
              virtual machine domains.

              The default udev(7) rules granting direct device  access  to  users  in  the  group
              bit-babbler,  enabling USB autosuspend when the device is idle, and invoking bbvirt
              to handle device hotplug for virtual machines.  These can be overridden by creating
              /etc/udev/rules.d/60-bit-babbler.rules and populating it with your own rules.

              The  libvirt  hook script needed to enable cold-plugging of already present devices
              into newly (re)started virtual machines.


       seedd(1), virsh(1).


       bbvirt was written by Ron <>.  You can send bug reports,  feature  requests,
       praise and complaints to

                                         January 2, 2018                                BBVIRT(1)