[PATCH] IB uverbs: add documentation file

USERSPACE VERBS ACCESS

  The ib_uverbs module, built by enabling CONFIG_INFINIBAND_USER_VERBS,

  enables direct userspace access to IB hardware via "verbs," as

  described in chapter 11 of the InfiniBand Architecture Specification.

  To use the verbs, the libibverbs library, available from

  <http://openib.org/>, is required.  libibverbs contains a

  device-independent API for using the ib_uverbs interface.

  libibverbs also requires appropriate device-dependent kernel and

  userspace driver for your InfiniBand hardware.  For example, to use

  a Mellanox HCA, you will need the ib_mthca kernel module and the

  libmthca userspace driver be installed.

User-kernel communication

  Userspace communicates with the kernel for slow path, resource

  management operations via the /dev/infiniband/uverbsN character

  devices.  Fast path operations are typically performed by writing

  directly to hardware registers mmap()ed into userspace, with no

  system call or context switch into the kernel.

  Commands are sent to the kernel via write()s on these device files.

  The ABI is defined in drivers/infiniband/include/ib_user_verbs.h.

  The structs for commands that require a response from the kernel

  contain a 64-bit field used to pass a pointer to an output buffer.

  Status is returned to userspace as the return value of the write()

  system call.

Resource management

  Since creation and destruction of all IB resources is done by

  commands passed through a file descriptor, the kernel can keep track

  of which resources are attached to a given userspace context.  The

  ib_uverbs module maintains idr tables that are used to translate

  between kernel pointers and opaque userspace handles, so that kernel

  pointers are never exposed to userspace and userspace cannot trick

  the kernel into following a bogus pointer.

  This also allows the kernel to clean up when a process exits and

  prevent one process from touching another process's resources.

Memory pinning

  Direct userspace I/O requires that memory regions that are potential

  I/O targets be kept resident at the same physical address.  The

  ib_uverbs module manages pinning and unpinning memory regions via

  get_user_pages() and put_page() calls.  It also accounts for the

  amount of memory pinned in the process's locked_vm, and checks that

  unprivileged processes do not exceed their RLIMIT_MEMLOCK limit.

  Pages that are pinned multiple times are counted each time they are

  pinned, so the value of locked_vm may be an overestimate of the

  number of pages pinned by a process.

/dev files

  To create the appropriate character device files automatically with

  udev, a rule like

    KERNEL="uverbs*", NAME="infiniband/%k"

  can be used.  This will create device nodes named

    /dev/infiniband/uverbs0

  and so on.  Since the InfiniBand userspace verbs should be safe for

  use by non-privileged processes, it may be useful to add an

  appropriate MODE or GROUP to the udev rule.