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Documentation / infiniband / user_verbs.txt




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Based on kernel version 3.13. Page generated on 2014-01-20 22:03 EST.

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://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	
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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