1 USERSPACE 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://openib.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
15 User-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
30 Resource 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
43 Memory 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.
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