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authorRoland Dreier <rolandd@cisco.com>2005-07-07 17:57:21 -0700
committerLinus Torvalds <torvalds@g5.osdl.org>2005-07-07 18:23:50 -0700
commit6f50142e4b092a469920a0008fc23121c3d99f2f (patch)
treee63c94bbcca7eef0745372c98812691cf97829ab /Documentation/infiniband
parent80c8ec2c04e539aac4e9810a46bc04c1b424b4dd (diff)
downloadlinux-6f50142e4b092a469920a0008fc23121c3d99f2f.tar.gz
[PATCH] IB uverbs: add documentation file
Add documentation for InfiniBand userspace verbs. Signed-off-by: Roland Dreier <rolandd@cisco.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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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.
+
+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.