From 4eea3efc13acccfb613571f604f17e0ec68e5bed Mon Sep 17 00:00:00 2001 From: Thomas Schwinge Date: Mon, 13 Dec 2010 20:22:52 +0100 Subject: ``Some'' Mach documentation. Parts have been rescued from 4b382d8daa5a9e2d54e78c18beeff76bc54dc16b:Mach/MachConcepts.mdwn. --- microkernel/mach/port.mdwn | 114 +++++++++++++++++++++++++++++++-------------- 1 file changed, 79 insertions(+), 35 deletions(-) (limited to 'microkernel/mach/port.mdwn') diff --git a/microkernel/mach/port.mdwn b/microkernel/mach/port.mdwn index af4a0c8d..ba2e22c2 100644 --- a/microkernel/mach/port.mdwn +++ b/microkernel/mach/port.mdwn @@ -1,41 +1,85 @@ -[[!meta copyright="Copyright © 2007, 2008 Free Software Foundation, Inc."]] +[[!meta copyright="Copyright © 2002, 2003, 2007, 2008, 2010 Free Software +Foundation, Inc."]] [[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable id="license" text="Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license -is included in the section entitled -[[GNU Free Documentation License|/fdl]]."]]"""]] - -Mach ports are [[capabilities|capability]]. - -A Mach port is a kernel queue. Each port has associated with -it a receive right and one or more send and send-once rights. -A queue can hold a number of messages. Once the queue is full, -the send blocks until their is space to enqueue the message -(this is interruptible via a timeout mechanism). - -A receive right designates a queue and authorizes the holder to -dequeue messages from the queue, and to create send and send-once -rights. - -Send and send-once rights designate a queue and authorize the -hold to enqueue messages (in the case of a send-once right, -a single message). Enqueuing a message is equivalent to -[[invoke|invoking]] a capability. - -Send and receive rights are named using local names. Each -task has associated with it a port [[address_space]]. A ports -are addressed via this table. Each task thus has its own -private [[naming_context]] for ports. - -Ports can be [[delegate]]d in an [[IPC]] message. When the -receiver dequeues the message, the right is made available -to it. - -A [[thread]] can only block receiving on a single port. To work -around this, the concept of a port set was introduced. A receive -right can be added to (at most) one port set. When a thread -receives from a port set, it dequeues from any of the ports that -has a message available. +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[Mach]] *port*s are [[capabilities|capability]], and are also essentially +similar to [[UNIX]] pipes. They are communication channels, implemented by +kernel queues. + +Each port has associated with it one *receive right* and one or more *send +right*s and *send-once right*s. That is, there is one receiver and one or more +senders -- a unidirectional communication channel. Only with the corresponding +port right, access to a port is possible; this is enforced by Mach. + +The kernel queue can hold a number of [[message]]s. Once the queue is full, +the send blocks until there is space to enqueue the message (this is +interruptible via a timeout mechanism). + +A receive right [[designates|designation]] a queue and authorizes the holder to +dequeue messages from the queue, and to create send and send-once rights. + +Send and send-once rights designate a queue and authorize the hold to enqueue +messages (in the case of a send-once right, a single message). Enqueuing a +message is equivalent to [[invoke|invoking]] a capability. + +Ports are automatically destroyed when there is no associated port right to +them. + +Mach knows what port rights belong to each task, but [[thread]]s that running +in the context of a task refer to ports by means of send and receive rights +that are named using local *port names*. These port names are plain integers, +like [[UNIX file descriptors|unix/file_descriptor]]. Only these local names +can be used by [[thread]]s for invoking operations on ports, threads do not +deal with port rights directly. + +For that, each task has associated with it a *port address_space*, or *port +name space*. All ports are addressed via this table. Each task thus has its +own private [[naming_context]] for port rights. + +So, the picture is that after obtaining a port send right, the client uses a +port name to send [[message]]s to the port, or exactly one message if it's a +send-once right. These messages are (probably) queued and when the server task +tries to receive messages by having a [[thread]] use its port receive right, it +gets the message(s). This is called [[IPC]]. + +Port rights themselvse can be [[delegate]]d in a [[message]], too. When the +receiver dequeues the message, the right is made available to it. + +The delivery of [[message]]s is reliable and strictly ordered. When a +[[thread]] sends messages *1* and *2*, it is guaranteed that the receiving +[[task]] will catch them in the same order. Of course, there can be +intermediate messages that are sent by other threads. + +Ports are objects that are implemented by the [[kernel]], and they are +kernel-protected resources. There is no way for a [[task]] to do anything with +a port unless it have corresponding port right. + +Due to this, ports are globally unique. This makes them ideal for constituting +system-wide *object references*. For example, the [[RPC]] system as used by +the GNU Hurd works by invoking *methods* on such object references. The +available methods are defined in [[hurd/interface]] files, and are processes by +the [[MIG]] tool. + +Invoking an operation on a port does not transfer the current execution control +to the receiver, but instead is an asynchronous operation. For this, and +especially in a [[RPC]] system, the sender may include a *reply port* using a +send-once right, and synchronize (block) on that one. + +A [[thread]] can only block receiving on a single port. To work around this, +the concept of a *port set* was introduced. A receive right can be added to +(at most) one port set. These port sets look like port receive rights, but +cannot be passed to other tasks, and there are additional operations for adding +and removing port receive rights. + +When a server process' thread receives from a port set, it dequeues exactly one +message from any of the ports that has a message available in its queue. + +This concept of port sets is also the facility that makes convenient +implementation of [[UNIX]]'s `select` [[system_call]] possible. -- cgit v1.2.3