linux/Documentation/infiniband/user_verbs.txt
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   1USERSPACE VERBS ACCESS
   2
   3  The ib_uverbs module, built by enabling CONFIG_INFINIBAND_USER_VERBS,
   4  enables direct userspace access to IB hardware via "verbs," as
   5  described in chapter 11 of the InfiniBand Architecture Specification.
   6
   7  To use the verbs, the libibverbs library, available from
   8  http://www.openfabrics.org/, is required.  libibverbs contains a
   9  device-independent API for using the ib_uverbs interface.
  10  libibverbs also requires appropriate device-dependent kernel and
  11  userspace driver for your InfiniBand hardware.  For example, to use
  12  a Mellanox HCA, you will need the ib_mthca kernel module and the
  13  libmthca userspace driver be installed.
  14
  15User-kernel communication
  16
  17  Userspace communicates with the kernel for slow path, resource
  18  management operations via the /dev/infiniband/uverbsN character
  19  devices.  Fast path operations are typically performed by writing
  20  directly to hardware registers mmap()ed into userspace, with no
  21  system call or context switch into the kernel.
  22
  23  Commands are sent to the kernel via write()s on these device files.
  24  The ABI is defined in drivers/infiniband/include/ib_user_verbs.h.
  25  The structs for commands that require a response from the kernel
  26  contain a 64-bit field used to pass a pointer to an output buffer.
  27  Status is returned to userspace as the return value of the write()
  28  system call.
  29
  30Resource management
  31
  32  Since creation and destruction of all IB resources is done by
  33  commands passed through a file descriptor, the kernel can keep track
  34  of which resources are attached to a given userspace context.  The
  35  ib_uverbs module maintains idr tables that are used to translate
  36  between kernel pointers and opaque userspace handles, so that kernel
  37  pointers are never exposed to userspace and userspace cannot trick
  38  the kernel into following a bogus pointer.
  39
  40  This also allows the kernel to clean up when a process exits and
  41  prevent one process from touching another process's resources.
  42
  43Memory pinning
  44
  45  Direct userspace I/O requires that memory regions that are potential
  46  I/O targets be kept resident at the same physical address.  The
  47  ib_uverbs module manages pinning and unpinning memory regions via
  48  get_user_pages() and put_page() calls.  It also accounts for the
  49  amount of memory pinned in the process's locked_vm, and checks that
  50  unprivileged processes do not exceed their RLIMIT_MEMLOCK limit.
  51
  52  Pages that are pinned multiple times are counted each time they are
  53  pinned, so the value of locked_vm may be an overestimate of the
  54  number of pages pinned by a process.
  55
  56/dev files
  57
  58  To create the appropriate character device files automatically with
  59  udev, a rule like
  60
  61    KERNEL=="uverbs*", NAME="infiniband/%k"
  62
  63  can be used.  This will create device nodes named
  64
  65    /dev/infiniband/uverbs0
  66
  67  and so on.  Since the InfiniBand userspace verbs should be safe for
  68  use by non-privileged processes, it may be useful to add an
  69  appropriate MODE or GROUP to the udev rule.
  70
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