1======================
2Userspace 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
17User-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
33Resource 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
47Memory 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.
76