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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://www.openfabrics.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.