Based on kernel version 4.2. Page generated on 2015-09-09 12:13 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.