76 lines
3.0 KiB
ReStructuredText
76 lines
3.0 KiB
ReStructuredText
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======================
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Userspace verbs access
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======================
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The ib_uverbs module, built by enabling CONFIG_INFINIBAND_USER_VERBS,
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enables direct userspace access to IB hardware via "verbs," as
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described in chapter 11 of the InfiniBand Architecture Specification.
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To use the verbs, the libibverbs library, available from
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https://github.com/linux-rdma/rdma-core, is required. libibverbs contains a
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device-independent API for using the ib_uverbs interface.
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libibverbs also requires appropriate device-dependent kernel and
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userspace driver for your InfiniBand hardware. For example, to use
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a Mellanox HCA, you will need the ib_mthca kernel module and the
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libmthca userspace driver be installed.
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User-kernel communication
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=========================
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Userspace communicates with the kernel for slow path, resource
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management operations via the /dev/infiniband/uverbsN character
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devices. Fast path operations are typically performed by writing
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directly to hardware registers mmap()ed into userspace, with no
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system call or context switch into the kernel.
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Commands are sent to the kernel via write()s on these device files.
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The ABI is defined in drivers/infiniband/include/ib_user_verbs.h.
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The structs for commands that require a response from the kernel
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contain a 64-bit field used to pass a pointer to an output buffer.
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Status is returned to userspace as the return value of the write()
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system call.
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Resource management
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===================
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Since creation and destruction of all IB resources is done by
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commands passed through a file descriptor, the kernel can keep track
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of which resources are attached to a given userspace context. The
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ib_uverbs module maintains idr tables that are used to translate
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between kernel pointers and opaque userspace handles, so that kernel
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pointers are never exposed to userspace and userspace cannot trick
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the kernel into following a bogus pointer.
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This also allows the kernel to clean up when a process exits and
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prevent one process from touching another process's resources.
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Memory pinning
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==============
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Direct userspace I/O requires that memory regions that are potential
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I/O targets be kept resident at the same physical address. The
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ib_uverbs module manages pinning and unpinning memory regions via
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get_user_pages() and put_page() calls. It also accounts for the
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amount of memory pinned in the process's pinned_vm, and checks that
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unprivileged processes do not exceed their RLIMIT_MEMLOCK limit.
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Pages that are pinned multiple times are counted each time they are
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pinned, so the value of pinned_vm may be an overestimate of the
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number of pages pinned by a process.
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/dev files
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==========
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To create the appropriate character device files automatically with
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udev, a rule like::
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KERNEL=="uverbs*", NAME="infiniband/%k"
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can be used. This will create device nodes named::
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/dev/infiniband/uverbs0
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and so on. Since the InfiniBand userspace verbs should be safe for
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use by non-privileged processes, it may be useful to add an
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appropriate MODE or GROUP to the udev rule.
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