docs: add blkdebug block driver documentation

The blkdebug block driver is undocumented.  Documenting it is worthwhile
since it offers powerful error injection features that are used by
qemu-iotests test cases.

This document will make it easier for people to learn about and use
blkdebug.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

docs/blkdebug.txt

+Block I/O error injection using blkdebug
+----------------------------------------
+Copyright (C) 2014 Red Hat Inc
+
+This work is licensed under the terms of the GNU GPL, version 2 or later.  See
+the COPYING file in the top-level directory.
+
+The blkdebug block driver is a rule-based error injection engine.  It can be
+used to exercise error code paths in block drivers including ENOSPC (out of
+space) and EIO.
+
+This document gives an overview of the features available in blkdebug.
+
+Background
+----------
+Block drivers have many error code paths that handle I/O errors.  Image formats
+are especially complex since metadata I/O errors during cluster allocation or
+while updating tables happen halfway through request processing and require
+discipline to keep image files consistent.
+
+Error injection allows test cases to trigger I/O errors at specific points.
+This way, all error paths can be tested to make sure they are correct.
+
+Rules
+-----
+The blkdebug block driver takes a list of "rules" that tell the error injection
+engine when to fail an I/O request.
+
+Each I/O request is evaluated against the rules.  If a rule matches the request
+then its "action" is executed.
+
+Rules can be placed in a configuration file; the configuration file
+follows the same .ini-like format used by QEMU's -readconfig option, and
+each section of the file represents a rule.
+
+The following configuration file defines a single rule:
+
+  $ cat blkdebug.conf
+  [inject-error]
+  event = "read_aio"
+  errno = "28"
+
+This rule fails all aio read requests with ENOSPC (28).  Note that the errno
+value depends on the host.  On Linux, see
+/usr/include/asm-generic/errno-base.h for errno values.
+
+Invoke QEMU as follows:
+
+  $ qemu-system-x86_64
+        -drive if=none,cache=none,file=blkdebug:blkdebug.conf:test.img,id=drive0 \
+        -device virtio-blk-pci,drive=drive0,id=virtio-blk-pci0
+
+Rules support the following attributes:
+
+  event - which type of operation to match (e.g. read_aio, write_aio,
+          flush_to_os, flush_to_disk).  See the "Events" section for
+          information on events.
+
+  state - (optional) the engine must be in this state number in order for this
+          rule to match.  See the "State transitions" section for information
+          on states.
+
+  errno - the numeric errno value to return when a request matches this rule.
+          The errno values depend on the host since the numeric values are not
+          standarized in the POSIX specification.
+
+  sector - (optional) a sector number that the request must overlap in order to
+           match this rule
+
+  once - (optional, default "off") only execute this action on the first
+         matching request
+
+  immediately - (optional, default "off") return a NULL BlockDriverAIOCB
+                pointer and fail without an errno instead.  This exercises the
+                code path where BlockDriverAIOCB fails and the caller's
+                BlockDriverCompletionFunc is not invoked.
+
+Events
+------
+Block drivers provide information about the type of I/O request they are about
+to make so rules can match specific types of requests.  For example, the qcow2
+block driver tells blkdebug when it accesses the L1 table so rules can match
+only L1 table accesses and not other metadata or guest data requests.
+
+The core events are:
+
+  read_aio - guest data read
+
+  write_aio - guest data write
+
+  flush_to_os - write out unwritten block driver state (e.g. cached metadata)
+
+  flush_to_disk - flush the host block device's disk cache
+
+See block/blkdebug.c:event_names[] for the full list of events.  You may need
+to grep block driver source code to understand the meaning of specific events.
+
+State transitions
+-----------------
+There are cases where more power is needed to match a particular I/O request in
+a longer sequence of requests.  For example:
+
+  write_aio
+  flush_to_disk
+  write_aio
+
+How do we match the 2nd write_aio but not the first?  This is where state
+transitions come in.
+
+The error injection engine has an integer called the "state" that always starts
+initialized to 1.  The state integer is internal to blkdebug and cannot be
+observed from outside but rules can interact with it for powerful matching
+behavior.
+
+Rules can be conditional on the current state and they can transition to a new
+state.
+
+When a rule's "state" attribute is non-zero then the current state must equal
+the attribute in order for the rule to match.
+
+For example, to match the 2nd write_aio:
+
+  [set-state]
+  event = "write_aio"
+  state = "1"
+  new_state = "2"
+
+  [inject-error]
+  event = "write_aio"
+  state = "2"
+  errno = "5"
+
+The first write_aio request matches the set-state rule and transitions from
+state 1 to state 2.  Once state 2 has been entered, the set-state rule no
+longer matches since it requires state 1.  But the inject-error rule now
+matches the next write_aio request and injects EIO (5).
+
+State transition rules support the following attributes:
+
+  event - which type of operation to match (e.g. read_aio, write_aio,
+          flush_to_os, flush_to_disk).  See the "Events" section for
+          information on events.
+
+  state - (optional) the engine must be in this state number in order for this
+          rule to match
+
+  new_state - transition to this state number
+
+Suspend and resume
+------------------
+Exercising code paths in block drivers may require specific ordering amongst
+concurrent requests.  The "breakpoint" feature allows requests to be halted on
+a blkdebug event and resumed later.  This makes it possible to achieve
+deterministic ordering when multiple requests are in flight.
+
+Breakpoints on blkdebug events are associated with a user-defined "tag" string.
+This tag serves as an identifier by which the request can be resumed at a later
+point.
+
+See the qemu-io(1) break, resume, remove_break, and wait_break commands for
+details.