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+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.
+ 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.
+ 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.
+ 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.
+ 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
+ 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.