Documentation/infiniband/user_verbs.rst

   1 ======================
   2 Userspace verbs access
   3 ======================
   4
   5   The ib_uverbs module, built by enabling CONFIG_INFINIBAND_USER_VERBS,
   6   enables direct userspace access to IB hardware via "verbs," as
   7   described in chapter 11 of the InfiniBand Architecture Specification.
   8
   9   To use the verbs, the libibverbs library, available from
  10   https://github.com/linux-rdma/rdma-core, is required. libibverbs contains a
  11   device-independent API for using the ib_uverbs interface.
  12   libibverbs also requires appropriate device-dependent kernel and
  13   userspace driver for your InfiniBand hardware.  For example, to use
  14   a Mellanox HCA, you will need the ib_mthca kernel module and the
  15   libmthca userspace driver be installed.
  16
  17 User-kernel communication
  18 =========================
  19
  20   Userspace communicates with the kernel for slow path, resource
  21   management operations via the /dev/infiniband/uverbsN character
  22   devices.  Fast path operations are typically performed by writing
  23   directly to hardware registers mmap()ed into userspace, with no
  24   system call or context switch into the kernel.
  25
  26   Commands are sent to the kernel via write()s on these device files.
  27   The ABI is defined in drivers/infiniband/include/ib_user_verbs.h.
  28   The structs for commands that require a response from the kernel
  29   contain a 64-bit field used to pass a pointer to an output buffer.
  30   Status is returned to userspace as the return value of the write()
  31   system call.
  32
  33 Resource management
  34 ===================
  35
  36   Since creation and destruction of all IB resources is done by
  37   commands passed through a file descriptor, the kernel can keep track
  38   of which resources are attached to a given userspace context.  The
  39   ib_uverbs module maintains idr tables that are used to translate
  40   between kernel pointers and opaque userspace handles, so that kernel
  41   pointers are never exposed to userspace and userspace cannot trick
  42   the kernel into following a bogus pointer.
  43
  44   This also allows the kernel to clean up when a process exits and
  45   prevent one process from touching another process's resources.
  46
  47 Memory pinning
  48 ==============
  49
  50   Direct userspace I/O requires that memory regions that are potential
  51   I/O targets be kept resident at the same physical address.  The
  52   ib_uverbs module manages pinning and unpinning memory regions via
  53   get_user_pages() and put_page() calls.  It also accounts for the
  54   amount of memory pinned in the process's pinned_vm, and checks that
  55   unprivileged processes do not exceed their RLIMIT_MEMLOCK limit.
  56
  57   Pages that are pinned multiple times are counted each time they are
  58   pinned, so the value of pinned_vm may be an overestimate of the
  59   number of pages pinned by a process.
  60
  61 /dev files
  62 ==========
  63
  64   To create the appropriate character device files automatically with
  65   udev, a rule like::
  66
  67     KERNEL=="uverbs*", NAME="infiniband/%k"
  68
  69   can be used.  This will create device nodes named::
  70
  71     /dev/infiniband/uverbs0
  72
  73   and so on.  Since the InfiniBand userspace verbs should be safe for
  74   use by non-privileged processes, it may be useful to add an
  75   appropriate MODE or GROUP to the udev rule.