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| -rw-r--r-- | Documentation/infiniband/user_verbs.txt | 69 |
1 files changed, 69 insertions, 0 deletions
diff --git a/Documentation/infiniband/user_verbs.txt b/Documentation/infiniband/user_verbs.txt new file mode 100644 index 00000000000..f847501e50b --- /dev/null +++ b/Documentation/infiniband/user_verbs.txt @@ -0,0 +1,69 @@ +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. |