USERSPACE VERBS ACCESS
  
    The ib_uverbs module, built by enabling CONFIG_INFINIBAND_USER_VERBS,
    enables direct userspace access to IB hardware via "verbs," as
    described in chapter 11 of the InfiniBand Architecture Specification.
  
    To use the verbs, the libibverbs library, available from

    http://www.openfabrics.org/, is required.  libibverbs contains a

    device-independent API for using the ib_uverbs interface.
    libibverbs also requires appropriate device-dependent kernel and
    userspace driver for your InfiniBand hardware.  For example, to use
    a Mellanox HCA, you will need the ib_mthca kernel module and the
    libmthca userspace driver be installed.
  
  User-kernel communication
  
    Userspace communicates with the kernel for slow path, resource
    management operations via the /dev/infiniband/uverbsN character
    devices.  Fast path operations are typically performed by writing
    directly to hardware registers mmap()ed into userspace, with no
    system call or context switch into the kernel.
  
    Commands are sent to the kernel via write()s on these device files.
    The ABI is defined in drivers/infiniband/include/ib_user_verbs.h.
    The structs for commands that require a response from the kernel
    contain a 64-bit field used to pass a pointer to an output buffer.
    Status is returned to userspace as the return value of the write()
    system call.
  
  Resource management
  
    Since creation and destruction of all IB resources is done by
    commands passed through a file descriptor, the kernel can keep track
    of which resources are attached to a given userspace context.  The
    ib_uverbs module maintains idr tables that are used to translate
    between kernel pointers and opaque userspace handles, so that kernel
    pointers are never exposed to userspace and userspace cannot trick
    the kernel into following a bogus pointer.
  
    This also allows the kernel to clean up when a process exits and
    prevent one process from touching another process's resources.
  
  Memory pinning
  
    Direct userspace I/O requires that memory regions that are potential
    I/O targets be kept resident at the same physical address.  The
    ib_uverbs module manages pinning and unpinning memory regions via
    get_user_pages() and put_page() calls.  It also accounts for the
    amount of memory pinned in the process's locked_vm, and checks that
    unprivileged processes do not exceed their RLIMIT_MEMLOCK limit.
  
    Pages that are pinned multiple times are counted each time they are
    pinned, so the value of locked_vm may be an overestimate of the
    number of pages pinned by a process.
  
  /dev files
  
    To create the appropriate character device files automatically with
    udev, a rule like

      KERNEL=="uverbs*", NAME="infiniband/%k"

  
    can be used.  This will create device nodes named
  
      /dev/infiniband/uverbs0
  
    and so on.  Since the InfiniBand userspace verbs should be safe for
    use by non-privileged processes, it may be useful to add an
    appropriate MODE or GROUP to the udev rule.