Debugging, Testing, and Tuning Device Drivers

This chapter presents an overview of the various tools that are provided to assist with testing, debugging, and tuning device drivers. This chapter provides information on the following subjects:

• Testing Drivers – Testing a driver can potentially impair a system's ability to function. Use of both serial connections and alternate kernels helps facilitate recovery from crashes.

• Debugging Tools – Integral debugging facilities enable you to exercise and observe driver features conveniently without having to run a separate debugger.

• Tuning Drivers – illumos provides facilities for measuring the performance of device drivers. Writing kernel statistics structures for your device exports continuous statistics as the device is running. If an area for performance improvement is determined, then the DTrace dynamic instrumentation tool can help determine any problems more precisely.

22.1. Testing Drivers

To avoid data loss and other problems, you should take special care when testing a new device driver. This section discusses various testing strategies. For example, setting up a separate system that you control through a serial connection is the safest way to test a new driver. You can load test modules with various kernel variable settings to test performance under different kernel conditions. Should your system crash, you should be prepared to restore backup data, analyze any crash dumps, and rebuild the device directory.

22.1.1. Enable the Deadman Feature to Avoid a Hard Hang

If your system is in a hard hang, then you cannot break into the debugger. If you enable the deadman feature, the system panics instead of hanging indefinitely. You can then use the kmdb(1) kernel debugger to analyze your problem.

The deadman feature checks every second whether the system clock is updating. If the system clock is not updating, then you are in an indefinite hang. If the system clock has not been updated for 50 seconds, the deadman feature induces a panic and puts you in the debugger.

1. Make sure you are capturing crash images with dumpadm(1M).

2. Set the snooping variable in the /etc/system file. See the system(4) man page for information on the /etc/system file.

   set snooping=1

3. Reboot the system so that the /etc/system file is read again and the snooping setting takes effect.

Note that any zones on your system inherit the deadman setting as well.

If your system hangs while the deadman feature is enabled, you should see output similar to the following example on your console:

panic[cpu1]/thread=30018dd6cc0: deadman: timed out after 9 seconds of
clock inactivity

panic: entering debugger (continue to save dump)

Inside the debugger, use the ::cpuinfo command to investigate why the clock interrupt was not able to fire and advance the system time.

22.1.2. Testing With a Serial Connection

Using a serial connection is a good way to test drivers. Use the tip(1) command to make a serial connection between a host system and a test system. With this approach, the tip window on the host console is used as the console of the test machine. See the tip(1) man page for additional information.

• Interactions with the test system and kernel debuggers can be monitored. For example, the window can keep a log of the session for use if the driver crashes the test system.

• The test machine can be accessed remotely by logging into a tip host machine and using tip(1) to connect to the test machine.

Although using a tip connection and a second machine are not required to debug an illumos device driver, this technique is still recommended.

Connect the host system to the test machine using serial port A on both machines.

This connection must be made with a null modem cable.

On the host system, make sure there is an entry in /etc/remote for the connection. See the remote(4) man page for details.

The terminal entry must match the serial port that is used. illumos comes with the correct entry for serial port B, but a terminal entry must be added for serial port A:

debug:\
        :dv=/dev/term/a:br#9600:el=^C^S^Q^U^D:ie=%$:oe=^D:

The baud rate must be set to 9600.

In a shell window on the host, run tip(1) and specify the name of the entry:

% tip debug
connected

The shell window is now a tip window with a connection to the console of the test machine.

Do not use STOP-A for SPARC machines or F1-A for x86 architecture machines on the host machine to stop the test machine. This action actually stops the host machine. To send a break to the test machine, type ~# in the tip window. Commands such as ~# are recognized only if these characters on first on the line. If the command has no effect, press either the Return key or Control-U.

Setting Up a Target System on the SPARC Platform

A quick way to set up the test machine on the SPARC platform is to unplug the keyboard before turning on the machine. The machine then automatically uses serial port A as the console.

Another way to set up the test machine is to use boot PROM commands to make serial port A the console. On the test machine, at the boot PROM ok prompt, direct console I/O to the serial line. To make the test machine always come up with serial port A as the console, set the environment variables: input-device and output-device.

ok setenv input-device ttya
ok setenv output-device ttya

The eeprom command can also be used to make serial port A the console. As superuser, execute the following commands to make the input-device and output-device parameters point to serial port A. The following example demonstrates the eeprom command.

# eeprom input-device=ttya
# eeprom output-device=ttya

The eeprom commands cause the console to be redirected to serial port A at each subsequent system boot.

Setting Up a Target System on the x86 Platform

On x86 platforms, use the eeprom command to make serial port A the console. This procedure is the same as the SPARC platform procedure. See Setting Up a Target System on the SPARC Platform. The eeprom command causes the console to switch to serial port A (COM1) during reboot.

x86 machines do not transfer console control to the tip connection until an early stage in the boot process unless the BIOS supports console redirection to a serial port. In SPARC machines, the tip connection maintains console control throughout the boot process.

22.1.3. Setting Up Test Modules

The system(4) file in the /etc directory enables you to set the value of kernel variables at boot time. With kernel variables, you can toggle different behaviors in a driver and take advantage of debugging features that are provided by the kernel. The kernel variables moddebug and kmem_flags, which can be very useful in debugging, are discussed later in this section. See also Enable the Deadman Feature to Avoid a Hard Hang.

Changes to kernel variables after boot are unreliable, because /etc/system is read only once when the kernel boots. After this file is modified, the system must be rebooted for the changes to take effect. If a change in the file causes the system not to work, boot with the ask (-a) option. Then specify /dev/null as the system file.

Kernel variables cannot be relied on to be present in subsequent releases.

Setting Kernel Variables

The set command changes the value of module or kernel variables. To set module variables, specify the module name and the variable:

set module_name:variable=value

For example, to set the variable test_debug in a driver that is named myTest, use set as follows:

% set myTest:test_debug=1

To set a variable that is exported by the kernel itself, omit the module name.

You can also use a bitwise OR operation to set a value, for example:

% set moddebug | 0x80000000

Loading and Unloading Test Modules

The commands modload(1M), modunload(1M), and modinfo(1M) can be used to add test modules, which is a useful technique for debugging and stress-testing drivers. These commands are generally not needed in normal operation, because the kernel automatically loads needed modules and unloads unused modules. The moddebug kernel variable works with these commands to provide information and set controls.

Using the modload Function

Use modload(1M) to force a module into memory. The modload command verifies that the driver has no unresolved references when that driver is loaded. Loading a driver does not necessarily mean that the driver can attach. When a driver loads successfully, the driver's _info(9E) entry point is called. The attach entry point is not necessarily called.

Using the modinfo Function

Use modinfo(1M) to confirm that the driver is loaded.

$ modinfo
 Id Loadaddr   Size Info Rev Module Name
  6 101b6000    732   -   1  obpsym (OBP symbol callbacks)
  7 101b65bd  1acd0 226   1  rpcmod (RPC syscall)
  7 101b65bd  1acd0 226   1  rpcmod (32-bit RPC syscall)
  7 101b65bd  1acd0   1   1  rpcmod (rpc interface str mod)
  8 101ce8dd  74600   0   1  ip (IP STREAMS module)
  8 101ce8dd  74600   3   1  ip (IP STREAMS device)
...
$ modinfo | grep mydriver
169 781a8d78   13fb   0   1  mydriver (Test Driver 1.5)

The number in the info field is the major number that has been chosen for the driver. The modunload(1M) command can be used to unload a module if the module ID is provided. The module ID is found in the left column of modinfo output.

Sometimes a driver does not unload as expected after a modunload is issued, because the driver is determined to be busy. This situation occurs when the driver fails detach(9E), either because the driver really is busy, or because the detach entry point is implemented incorrectly.

Using modunload

To remove all of the currently unused modules from memory, run modunload(1M) with a module ID of 0:

# modunload -i 0

Setting the moddebug Kernel Variable

The moddebug kernel variable controls the module loading process. The possible values of moddebug are:

0x80000000

Prints messages to the console when loading or unloading modules.

0x40000000

Gives more detailed error messages.

0x20000000

Prints more detail when loading or unloading, such as including the address and size.

0x00001000

No auto-unloading drivers. The system does not attempt to unload the device driver when the system resources become low.

0x00000080

No auto-unloading streams. The system does not attempt to unload the STREAMS module when the system resources become low.

0x00000010

No auto-unloading of kernel modules of any type.

0x00000001

If running with kmdb, moddebug causes a breakpoint to be executed and a return to kmdb immediately before each module's _init routine is called. This setting also generates additional debug messages when the module's _info and _fini routines are executed.

Setting kmem_flags Debugging Flags

The kmem_flags kernel variable enables debugging features in the kernel's memory allocator. Set kmem_flags to 0xf to enable the allocator's debugging features. These features include runtime checks to find the following code conditions:

• Writing to a buffer after the buffer is freed

• Using memory before the memory is initialized

• Writing past the end of a buffer

The Modular Debugger Guide describes how to use the kernel memory allocator to analyze such problems.

Testing and developing with kmem_flags set to 0xf can help detect latent memory corruption bugs. Because setting kmem_flags to 0xf changes the internal behavior of the kernel memory allocator, you should thoroughly test without kmem_flags as well.

22.1.4. Avoiding Data Loss on a Test System

A driver bug can sometimes render a system incapable of booting. By taking precautions, you can avoid system reinstallation in this event, as described in this section.

Back Up Critical System Files

A number of driver-related system files are difficult, if not impossible, to reconstruct. Files such as /etc/name_to_major, /etc/driver_aliases, /etc/driver_classes, and /etc/minor_perm can be corrupted if the driver crashes the system during installation. See the add_drv(1M) man page.

To be safe, make a backup copy of the root file system after the test machine is in the proper configuration. If you plan to modify the /etc/system file, make a backup copy of the file before making modifications.

To avoid rendering a system inoperable, you should boot from a copy of the kernel and associated binaries rather than from the default kernel.

Make a copy of the drivers in /platform/*.

# cp -r /platform/`uname -i`/kernel /platform/`uname -i`/kernel.test

Place the driver module in /platform/`uname -i`/kernel.test/drv.

Boot the alternate kernel instead of the default kernel.

After you have created and stored the alternate kernel, you can boot this kernel in a number of ways.

• You can boot the alternate kernel by rebooting:

  # reboot -- kernel.test/unix

• On a SPARC-based system, you can also boot from the PROM:

  ok boot kernel.test/sparcv9/unix

  To boot with the kmdb debugger, use the -k option as described in Getting Started With the Modular Debugger.

• On an x86-based system, when the Select (b)oot or (i)nterpreter: message is displayed in the boot process, type the following:

  boot kernel.test/unix

ok boot kernel.test/sparcv9/unix
Rebooting with command: boot kernel.test/sparcv9/unix
Boot device: /sbus@1f,0/espdma@e,8400000/esp@e,8800000/sd@0,0:a File and \
    args:
kernel.test/sparcv9/unix

ok boot -a
Rebooting with command: boot -a
Boot device: /sbus@1f,0/espdma@e,8400000/esp@e,8800000/sd@0,0:a File and \
args: -a
Enter filename [kernel/sparcv9/unix]: kernel.test/sparcv9/unix
Enter default directory for modules
[/platform/sun4u/kernel.test /kernel /usr/kernel]: <CR>
Name of system file [etc/system]: <CR>
SunOS Release 5.10 Version Generic 64-bit
Copyright 1983-2002 Sun Microsystems, Inc. All rights reserved.
root filesystem type [ufs]: <CR>
Enter physical name of root device
[/sbus@1f,0/espdma@e,8400000/esp@e,8800000/sd@0,0:a]: <CR>

Consider Alternative Back-Up Plans

If the system is attached to a network, the test machine can be added as a client of a server. If a problem occurs, the system can be booted from the network. The local disks can then be mounted, and any fixes can be made. Alternatively, the system can be booted directly from the illumos system CD-ROM.

Another way to recover from disaster is to have another bootable root file system. Use format(1M) to make a partition that is the exact size of the original. Then use dd(1M) to copy the bootable root file system. After making a copy, run fsck(1M) on the new file system to ensure its integrity.

Subsequently, if the system cannot boot from the original root partition, boot the backup partition. Use dd(1M) to copy the backup partition onto the original partition. You might have a situation where the system cannot boot even though the root file system is undamaged. For example, the damage might be limited to the boot block or the boot program. In such a case, you can boot from the backup partition with the ask (-a) option. You can then specify the original file system as the root file system.

Capture System Crash Dumps

When a system panics, the system writes an image of kernel memory to the dump device. The dump device is by default the most suitable swap device. The dump is a system crash dump, similar to core dumps generated by applications. On rebooting after a panic, savecore(1M) checks the dump device for a crash dump. If a dump is found, savecore makes a copy of the kernel's symbol table, which is called unix.n. The savecore utility then dumps a core file that is called vmcore.n in the core image directory. By default, the core image directory is /var/crash/machine_name. If /var/crash has insufficient space for a core dump, the system displays the needed space but does not actually save the dump. The mdb(1) debugger can then be used on the core dump and the saved kernel.

In most illumos distributions, crash dump is enabled by default. The dumpadm(1M) command is used to configure system crash dumps. Use the dumpadm command to verify that crash dumps are enabled and to determine the location of core files that have been saved.

You can prevent the savecore utility from filling the file system. Add a file that is named minfree to the directory in which the dumps are to be saved. In this file, specify the number of kilobytes to remain free after savecore has run. If insufficient space is available, the core file is not saved.

22.1.5. Recovering the Device Directory

Damage to the /devices and /dev directories can occur if the driver crashes during attach(9E). If either directory is damaged, you can rebuild the directory by booting the system and running fsck(1M) to repair the damaged root file system. The root file system can then be mounted. Recreate the /devices and /dev directories by running devfsadm(1M) and specifying the /devices directory on the mounted disk.

The following example shows how to repair a damaged root file system on a SPARC system. In this example, the damaged disk is /dev/dsk/c0t3d0s0, and an alternate boot disk is /dev/dsk/c0t1d0s0.

ok boot disk1
...
Rebooting with command: boot kernel.test/sparcv9/unix
Boot device: /sbus@1f,0/espdma@e,8400000/esp@e,8800000/sd@31,0:a File and \
    args:
kernel.test/sparcv9/unix
...
# fsck /dev/dsk/c0t3d0s0** /dev/dsk/c0t3d0s0
** Last Mounted on /
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
1478 files, 9922 used, 29261 free
     (141 frags, 3640 blocks, 0.4% fragmentation)
# mount /dev/dsk/c0t3d0s0 /mnt
# devfsadm -r /mnt

A fix to the /devices and /dev directories can allow the system to boot while other parts of the system are still corrupted. Such repairs are only a temporary fix to save information, such as system crash dumps, before reinstalling the system.

22.2. Debugging Tools

• The kmdb(1) kernel debugger provides typical runtime debugger facilities, such as breakpoints, watch points, and single-stepping. The kmdb debugger supersedes kadb, which was available in previous releases. The commands that were previously available from kadb are used in kmdb, in addition to new functionality. Where kadb could only be loaded at boot time, kmdb can be loaded at any time. The kmdb debugger is preferred for live, interactive debugging due to its execution controls.

• The mdb(1) modular debugger is more limited than kmdb as a real-time debugger, but mdb has rich facilities for postmortem debugging.

The kmdb and mdb debuggers mostly share the same user interface. Many debugging techniques therefore can be applied with the same commands in both tools. Both debuggers support macros, dcmds, and dmods. A dcmd (pronounced dee-command) is a routine in the debugger that can access any of the properties of the current target program. A dcmd can be dynamically loaded at runtime. A dmod, which is short for debugger module, is a package of dcmds that can be loaded to provide non-standard behavior.

Both mdb and kmdb are backward-compatible with legacy debuggers such as adb and kadb. The mdb debugger can execute all of the macros that are available to kmdb as well as any legacy user-defined macros for adb. See the Modular Debugger Guide for information about where to find standard macro sets.

22.2.1. Postmortem Debugging

Postmortem analysis offers numerous advantages to driver developers. More than one developer can examine a problem in parallel. Multiple instances of the debugger can be used simultaneously on a single crash dump. The analysis can be performed offline so that the crashed system can be returned to service, if possible. Postmortem analysis enables the use of user-developed debugger functionality in the form of dmods. Dmods can bundle functionality that would be too memory-intensive for real-time debuggers, such as kmdb.

When a system panics while kmdb is loaded, control is passed to the debugger for immediate investigation. If kmdb does not seem appropriate for analyzing the current problem, a good strategy is to use :c to continue execution and save the crash dump. When the system reboots, you can perform postmortem analysis with mdb on the saved crash dump. This process is analogous to debugging an application crash from a process core file.

In earlier versions of the Solaris operating system, adb(1) was the recommended tool for postmortem analysis. In the current illumos releases, mdb(1) is the recommended tool for postmortem analysis. The mdb feature set surpasses the set of commands from the legacy crash(1M) utility. The crash utility is no longer available in illumos.

22.2.2. Using the kmdb Kernel Debugger

• Control of kernel execution

• Inspection of the kernel state

• Live modifications to the code

This section assumes that you are already familiar with the kmdb debugger. The focus in this section is on kmdb capabilities that are useful in device driver design. To learn how to use kmdb in detail, refer to the kmdb(1) man page and to the Modular Debugger Guide. If you are familiar with kadb, refer to the kadb(1M) man page for the major differences between kadb and kmdb.

The kmdb debugger can be loaded and unloaded at will. Instructions for loading and unloading kmdb are in the Modular Debugger Guide. For safety and convenience, booting with an alternate kernel is highly encouraged. The boot process is slightly different between the SPARC platform and the x86 platform, as described in this section.

By default, kmdb uses the CPU ID as the prompt when kmdb is running. In the examples in this chapter [0] is used as the prompt unless otherwise noted.

Booting kmdb With an Alternate Kernel on the SPARC Platform

Use either of the following commands to boot a SPARC system with both kmdb and an alternate kernel:

boot kmdb -D kernel.test/sparcv9/unix 
boot kernel.test/sparcv9/unix -k

Booting kmdb With an Alternate Kernel on the x86 Platform

Use either of the following commands to boot an x86 system with both kmdb and an alternate kernel:

b kmdb -D kernel.test/unix 
b kernel.test/unix -k

Setting Breakpoints in kmdb

Use the bp command to set a breakpoint, as shown in the following example.

[0]> myModule`myBreakpointLocation::bp

If the target module has not been loaded, then an error message that indicates this condition is displayed, and the breakpoint is not created. In this case you can use a deferred breakpoint. A deferred breakpoint activates automatically when the specified module is loaded. Set a deferred breakpoint by specifying the target location after the bp command. The following example demonstrates a deferred breakpoint.

[0]>::bp myModule`myBreakpointLocation

For more information on using breakpoints, see the Modular Debugger Guide. You can also get help by typing either of the following two lines:

> ::help bp
> ::bp dcmd

kmdb Macros for Driver Developers

The kmdb(1M) debugger supports macros that can be used to display kernel data structures. Use $M to display kmdb macros. Macros are used in the form:

[ address ] $<macroname

Neither the information displayed by these macros nor the format in which the information is displayed, constitutes an interface. Therefore, the information and format can change at any time.

The kmdb macros in the following table are particularly useful to developers of device drivers. For convenience, legacy macro names are shown where applicable.

The ::devinfo dcmd displays a node state that can have one of the following values:

DS_ATTACHED

The driver's attach(9E) routine returned successfully.

DS_BOUND

The node is bound to a driver, but the driver's probe(9E) routine has not yet been called.

DS_INITIALIZED

The parent nexus has assigned a bus address for the driver. The implementation-specific initializations have been completed. The driver's probe(9E) routine has not yet been called at this point.

DS_LINKED

The device node has been linked into the kernel's device tree, but the system has not yet found a driver for this node.

DS_PROBED

The driver's probe(9E) routine returned successfully.

DS_READY

The device is fully configured.

22.2.3. Using the mdb Modular Debugger

• Live operating system components

• Operating system crash dumps

• User processes

• User process core dumps

• Object files

The mdb debugger provides sophisticated debugging support for analyzing kernel problems. This section provides an overview of mdb features. For a complete discussion of mdb, refer to the Modular Debugger Guide.

Although mdb can be used to alter live kernel state, mdb lacks the kernel execution control that is provided by kmdb. As a result kmdb is preferred for runtime debugging. The mdb debugger is used more for static situations.

The prompt for mdb is >.

Getting Started With the Modular Debugger

The mdb debugger provides an extensive programming API for implementing debugger modules so that driver developers can implement custom debugging support. The mdb debugger also provides many usability features, such as command-line editing, command history, an output pager, and online help.

The adb macros should no longer be used. That functionality has largely been superseded by the dcmds in mdb.

The mdb debugger provides a rich set of modules and dcmds. With these tools, you can debug the illumos kernel, any associated modules, and device drivers. These facilities enable you to perform tasks such as:

• Formulate complex debugging queries

• Locate all the memory allocated by a particular thread

• Print a visual picture of a kernel STREAM

• Determine what type of structure a particular address refers to

• Locate leaked memory blocks in the kernel

• Analyze memory to locate stack traces

• Assemble dcmds into modules called dmods for creating customized operations

To get started, switch to the crash directory and type mdb, specifying a system crash dump, as illustrated in the following example.

% cd /var/crash/testsystem
% ls
bounds     unix.0    vmcore.0
% mdb unix.0 vmcore.0
Loading modules: [ unix krtld genunix ufs_log ip usba s1394 cpc nfs ]
> ::status
debugging crash dump vmcore.0 (64-bit) from testsystem
operating system: 5.10 Generic (sun4u)
panic message: zero
dump content: kernel pages only

When mdb responds with the > prompt, you can run commands.

To examine the running kernel on a live system, run mdb from the system prompt as follows.

# mdb -k
Loading modules: [ unix krtld genunix ufs_log ip usba s1394 ptm cpc ipc nfs ]
> ::status
debugging live kernel (64-bit) on testsystem
operating system: 5.10 Generic (sun4u)

22.2.4. Useful Debugging Tasks With kmdb and mdb

This section provides examples of useful debugging tasks. The tasks in this section can be performed with either mdb or kmdb unless specifically noted. This section assumes a basic knowledge of the use of kmdb and mdb. Note that the information presented here is dependent on the type of system used. A Sun Blade^TM 100 workstation running the 64-bit kernel was used to produce these examples.

Because irreversible destruction of data can result from modifying data in kernel structures, you should exercise extreme caution. Do not modify or rely on data in structures that are not part of the illumos DDI. See the Intro(9S) man page for information on structures that are part of the illumos DDI.

Exploring System Registers With kmdb

The kmdb debugger can display machine registers as a group or individually. To display all registers as a group, use $r as shown in the following example.

[0]: $r

g0    0                                 l0      0
g1    100130a4      debug_enter         l1      edd00028
g2    10411c00      tsbmiss_area+0xe00  l2      10449c90
g3    10442000      ti_statetbl+0x1ba   l3      1b
g4    3000061a004                       l4      10474400     ecc_syndrome_tab+0x80
g5    0                                 l5      3b9aca00
g6    0                                 l6      0
g7    2a10001fd40                       l7      0
o0    0                                 i0      0
o1    c                                 i1      10449e50
o2    20                                i2      0
o3    300006b2d08                       i3      10
o4    0                                 i4      0
o5    0                                 i5      b0
sp    2a10001b451                       fp      2a10001b521
o7    1001311c      debug_enter+0x78    i7      1034bb24     zsa_xsint+0x2c4
y     0
tstate: 1604  (ccr=0x0, asi=0x0, pstate=0x16, cwp=0x4)
pstate: ag:0 ie:1 priv:1 am:0 pef:1 mm:0 tle:0 cle:0 mg:0 ig:0
winreg: cur:4 other:0 clean:7 cansave:1 canrest:5 wstate:14
tba   0x10000000
pc    edd000d8 edd000d8:        ta      %icc,%g0 + 125
npc   edd000dc edd000dc:        nop

The debugger exports each register value to a variable with the same name as the register. If you read the variable, the current value of the register is returned. If you write to the variable, the value of the associated machine register is changed. The following example changes the value of the %o0 register from 0 to 1 on an x86 machine.

[0]> <eax=K
        c1e6e0f0
[0]> 0>eax
[0]> <eax=K
        0
[0]>  c1e6e0f0>eax

If you need to inspect the registers of a different processor, you can use the ::cpuregs dcmd. The ID of the processor to be examined can be supplied as either the address to the dcmd or as the value of the -c option, as shown in the following example.

[0]> 0::cpuregs
   %cs = 0x0158            %eax = 0xc1e6e0f0 kmdbmod`kaif_dvec
   %ds = 0x0160            %ebx = 0x00000000

The following example switches from processor 0 to processor 3 on a SPARC machine. The %g3 register is inspected and then cleared. To confirm the new value, %g3 is read again.

[0]> 3::switch
[3]> <g3=K
        24
[3]> 0>g3
[3]> <g3
        0

Detecting Kernel Memory Leaks

The ::findleaks dcmd provides powerful, efficient detection of memory leaks in kernel crash dumps. The full set of kernel-memory debugging features must be enabled for ::findleaks to be effective. For more information, see Setting kmem_flags Debugging Flags. Run ::findleaks during driver development and testing to detect code that leaks memory, thus wasting kernel resources. See Modular Debugger Guide for a complete discussion of ::findleaks.

Code that leaks kernel memory can render the system vulnerable to denial-of-service attacks.

Writing Debugger Commands With mdb

The mdb debugger provides a powerful API for implementing debugger facilities that you customize to debug your driver. The Modular Debugger Guide explains the programming API in detail.

The SUNWmdbdm package installs sample mdb source code in the directory /usr/demo/mdb. You can use mdb to automate lengthy debugging chores or help to validate that your driver is behaving properly. You can also package your mdb debugging modules with your driver product. With packaging, these facilities are available to service personnel at a customer site.

Obtaining Kernel Data Structure Information

The illumos kernel provides data type information in structures that can be inspected with either kmdb or mdb.

The kmdb and mdb dcmds can be used only with objects that contain compressed symbolic debugging information that has been designed for use with mdb. This information is currently available only for certain illumos kernel modules. The SUNWzlib package must be installed to process the symbolic debugging information.

The following example demonstrates how to display the data in the scsi_pkt structure.

> 7079ceb0::print -t 'struct scsi_pkt'
{
    opaque_t pkt_ha_private = 0x7079ce20
    struct scsi_address pkt_address = {
        struct scsi_hba_tran *a_hba_tran = 0x70175e68
        ushort_t a_target = 0x6
        uchar_t a_lun = 0
        uchar_t a_sublun = 0
    }
    opaque_t pkt_private = 0x708db4d0
    int (*)() *pkt_comp = sd_intr
    uint_t pkt_flags = 0
    int pkt_time = 0x78
    uchar_t *pkt_scbp = 0x7079ce74
    uchar_t *pkt_cdbp = 0x7079ce64
    ssize_t pkt_resid = 0
    uint_t pkt_state = 0x37
    uint_t pkt_statistics = 0
    uchar_t pkt_reason = 0
}

The size of a data structure can be useful in debugging. Use the ::sizeof dcmd to obtain the size of a structure, as shown in the following example.

> ::sizeof struct scsi_pkt
sizeof (struct scsi_pkt) = 0x58

The address of a specific member within a structure is also useful in debugging. Several methods are available for determining a member's address.

Use the ::offsetof dcmd to obtain the offset for a given member of a structure, as in the following example.

> ::offsetof struct scsi_pkt pkt_state
offsetof (struct pkt_state) = 0x48

Use the ::print dcmd with the -a option to display the addresses of all members of a structure, as in the following example.

> ::print -a struct scsi_pkt
{
    0 pkt_ha_private
    8 pkt_address {
    ...
    }
    18 pkt_private
    ...
}

If an address is specified with ::print in conjunction with the -a option, the absolute address for each member is displayed.

> 10000000::print -a struct scsi_pkt
{
    10000000 pkt_ha_private
    10000008 pkt_address {
    ...
    }
    10000018 pkt_private
    ...
}

The ::print, ::sizeof and ::offsetof dcmds enable you to debug problems when your driver interacts with the illumos kernel.

This facility provides access to raw kernel data structures. You can examine any structure whether or not that structure appears as part of the DDI. Therefore, you should refrain from relying on any data structure that is not explicitly part of the DDI.

These dcmds should be used only with objects that contain compressed symbolic debugging information that has been designed for use with mdb. Symbolic debugging information is currently available for certain illumos kernel modules only. The SUNWzlib (32-bit) or SUNWzlibx (64-bit) decompression software must be installed to process the symbolic debugging information. The kmdb debugger can process symbolic type data with or without the SUNWzlib or SUNWzlibx packages.

Obtaining Device Tree Information

The mdb debugger provides the ::prtconf dcmd for displaying the kernel device tree. The output of the ::prtconf dcmd is similar to the output of the prtconf(1M) command.

> ::prtconf
300015d3e08      SUNW,Sun-Blade-100
    300015d3c28      packages (driver not attached)
        300015d3868      SUNW,builtin-drivers (driver not attached)
        300015d3688      deblocker (driver not attached)
        300015d34a8      disk-label (driver not attached)
        300015d32c8      terminal-emulator (driver not attached)
        300015d30e8      obp-tftp (driver not attached)
        300015d2f08      dropins (driver not attached)
        300015d2d28      kbd-translator (driver not attached)
        300015d2b48      ufs-file-system (driver not attached)
    300015d3a48      chosen (driver not attached)
    300015d2968      openprom (driver not attached)

You can display the node by using a macro, such as the ::devinfo dcmd, as shown in the following example.

> 300015d3e08::devinfo
300015d3e08      SUNW,Sun-Blade-100
        System properties at 0x300015abdc0:
            name='relative-addressing' type=int items=1
                value=00000001
            name='MMU_PAGEOFFSET' type=int items=1
                value=00001fff
            name='MMU_PAGESIZE' type=int items=1
                value=00002000
            name='PAGESIZE' type=int items=1
                value=00002000
        Driver properties at 0x300015abe00:
            name='pm-hardware-state' type=string items=1
                value='no-suspend-resume'

Use ::prtconf to see where your driver has attached in the device tree, and to display device properties. You can also specify the verbose (-v) flag to ::prtconf to display the properties for each device node, as follows.

> ::prtconf -v
DEVINFO          NAME
300015d3e08      SUNW,Sun-Blade-100
        System properties at 0x300015abdc0:
            name='relative-addressing' type=int items=1
                value=00000001
            name='MMU_PAGEOFFSET' type=int items=1
                value=00001fff
            name='MMU_PAGESIZE' type=int items=1
                value=00002000
            name='PAGESIZE' type=int items=1
                value=00002000
        Driver properties at 0x300015abe00:
            name='pm-hardware-state' type=string items=1
                value='no-suspend-resume'
        ...
        300015ce798      pci10b9,5229, instance #0
                Driver properties at 0x300015ab980:
                    name='target2-dcd-options' type=any items=4
                        value=00.00.00.a4
                    name='target1-dcd-options' type=any items=4
                        value=00.00.00.a2
                    name='target0-dcd-options' type=any items=4
                        value=00.00.00.a4

Another way to locate instances of your driver is the ::devbindings dcmd. Given a driver name, the command displays a list of all instances of the named driver as demonstrated in the following example.

> ::devbindings dad
300015ce3d8      ide-disk (driver not attached)
300015c9a60      dad, instance #0
        System properties at 0x300015ab400:
            name='lun' type=int items=1
                value=00000000
            name='target' type=int items=1
                value=00000000
            name='class_prop' type=string items=1
                value='ata'
            name='type' type=string items=1
                value='ata'
            name='class' type=string items=1
                value='dada'
...
300015c9880      dad, instance #1
        System properties at 0x300015ab080:
            name='lun' type=int items=1
                value=00000000
            name='target' type=int items=1
                value=00000002
            name='class_prop' type=string items=1
                value='ata'
            name='type' type=string items=1
                value='ata'
            name='class' type=string items=1
                value='dada'

Retrieving Driver Soft State Information

A common problem when debugging a driver is retrieving the soft state for a particular driver instance. The soft state is allocated with the ddi_soft_state_zalloc(9F) routine. The driver can obtain the soft state through ddi_get_soft_state(9F). The name of the soft state pointer is the first argument to ddi_soft_state_init(9F)). With the name, you can use mdb to retrieve the soft state for a particular driver instance through the ::softstate dcmd:

> *bst_state::softstate 0x3
702b7578

In this case, ::softstate is used to fetch the soft state for instance 3 of the bst sample driver. This pointer references a bst_soft structure that is used by the driver to track state for this instance.

Modifying Kernel Variables

You can use both kmdb and mdb to modify kernel variables or other kernel state. Kernel state modification with mdb should be done with care, because mdb does not stop the kernel before making modifications. Groups of modifications can be made atomically by using kmdb, because kmdb stops the kernel before allowing access by the user. The mdb debugger is capable of making single atomic modifications only.

• w – Writes the lowest two bytes of the value of each expression to the target beginning at the location specified by dot

• W – Writes the lowest 4 bytes of the value of each expression to the target beginning at the location specified by dot

• Z – Write the complete 8 bytes of the value of each expression to the target beginning at the location specified by dot

Use the ::sizeof dcmd to determine the size of the variable to be modified.

The following example overwrites the value of moddebug with the value 0x80000000.

> moddebug/W 0x80000000
    moddebug:       0 = 0x80000000

22.3. Tuning Drivers

illumos provides kernel statistics structures so that you can implement counters for your driver. The DTrace facility enables you to analyze performance in real time. This section presents the following topics on device performance:

• Kernel Statistics – illumos provides a set of data structures and functions for capturing performance statistics in the kernel. Kernel statistics (called kstats) enable your driver to export continuous statistics while the system is running. The kstat data is handled programmatically by using the kstat functions.

• DTrace for Dynamic Instrumentation – DTrace enables you to add instrumentation to your driver dynamically so that you can perform tasks like analyzing the system and measuring performance. DTrace takes advantage of predefined kstat structures.

22.3.1. Kernel Statistics

To assist in performance tuning, the illumos kernel provides the kstat(3KSTAT) facility. The kstat facility provides a set of functions and data structures for device drivers and other kernel modules to export module-specific kernel statistics.

A kstat is a data structure for recording quantifiable aspects of a device's usage. A kstat is stored as a null-terminated linked list. Each kstat has a common header section and a type-specific data section. The header section is defined by the kstat_t structure.

The article “Using kstat From Within a Program in the Solaris OS” on the Sun Developer Network at http://developers.sun.com/solaris/articles/kstat_api.html provides two practical examples on how to use the kstat(3KSTAT) and libkstat(3LIB) APIs to extract metrics from illumos. The examples include “Walking Through All the kstat” and “Getting NIC kstat Output Using the Java Platform.”

Kernel Statistics Structure Members

The members of a kstat structure are:

ks_class[KSTAT_STRLEN]

Categorizes the kstat type as bus, controller, device_error, disk, hat, kmem_cache, kstat, misc, net, nfs, pages, partition, rps, ufs, vm, or vmem.

ks_crtime

Time at which the kstat was created. ks_crtime is commonly used in calculating rates of various counters.

ks_data

Points to the data section for the kstat.

ks_data_size

Total size of the data section in bytes.

ks_instance

The instance of the kernel module that created this kstat. ks_instance is combined with ks_module and ks_name to give the kstat a unique, meaningful name.

ks_kid

Unique ID for the kstat.

ks_module[KSTAT_STRLEN]

Identifies the kernel module that created this kstat. ks_module is combined with ks_instance and ks_name to give the kstat a unique, meaningful name. KSTAT_STRLEN sets the maximum length of ks_module.

ks_name[KSTAT_STRLEN]

A name assigned to the kstat in combination with ks_module and ks_instance. KSTAT_STRLEN sets the maximum length of ks_module.

ks_ndata

Indicates the number of data records for those kstat types that support multiple records: KSTAT_TYPE_RAW, KSTAT_TYPE_NAMED, and KSTAT_TYPE_TIMER

ks_next

Points to next kstat in the chain.

ks_resv

A reserved field.

ks_snaptime

The timestamp for the last data snapshot, useful in calculating rates.

ks_type

The data type, which can be KSTAT_TYPE_RAW for binary data, KSTAT_TYPE_NAMED for name/value pairs, KSTAT_TYPE_INTR for interrupt statistics, KSTAT_TYPE_IO for I/O statistics, and KSTAT_TYPE_TIMER for event timers.

Kernel Statistics Structures

The structures for the different kinds of kstats are:

kstat(9S)

Each kernel statistic (kstat) that is exported by device drivers consists of a header section and a data section. The kstat(9S) structure is the header portion of the statistic.

kstat_intr(9S)

Structure for interrupt kstats. The types of interrupts are:

• Hard interrupt – Sourced from the hardware device itself

• Soft interrupt – Induced by the system through the use of some system interrupt source

• Watchdog interrupt – Induced by a periodic timer call

• Spurious interrupt – An interrupt entry point was entered but there was no interrupt to service

• Multiple service – An interrupt was detected and serviced just prior to returning from any of the other types

Drivers generally report only claimed hard interrupts and soft interrupts from their handlers, but measurement of the spurious class of interrupts is useful for auto-vectored devices to locate any interrupt latency problems in a particular system configuration. Devices that have more than one interrupt of the same type should use multiple structures.

kstat_io(9S)

Structure for I/O kstats.

kstat_named(9S)

Structure for named kstats. A named kstat is an array of name-value pairs. These pairs are kept in the kstat_named structure.

Kernel Statistics Functions

The functions for using kstats are:

kstat_create(9F)

Allocate and initialize a kstat(9S) structure.

kstat_delete(9F)

Remove a kstat from the system.

kstat_install(9F)

Add a fully initialized kstat to the system.

kstat_named_init(9F), kstat_named_setstr(9F)

Initialize a named kstat. kstat_named_setstr associates str, a string, with the named kstat pointer.

kstat_queue(9F)

A large number of I/O subsystems have at least two basic queues of transactions to be managed. One queue is for transactions that have been accepted for processing but for which processing has yet to begin. The other queue is for transactions that are actively being processed but not yet done. For this reason, two cumulative time statistics are kept:wait time and run time. Wait time is prior to service. Run time is during the service. The kstat_queue family of functions manages these times based on the transitions between the driver wait queue and run queue:

• kstat_runq_back_to_waitq(9F)

• kstat_runq_enter(9F)

• kstat_runq_exit(9F)

• kstat_waitq_enter(9F)

• kstat_waitq_exit(9F)

• kstat_waitq_to_runq(9F)

Kernel Statistics for illumos Ethernet Drivers

The kstat interface described in the following table is an effective way to obtain Ethernet physical layer statistics from the driver. Ethernet drivers should export these statistics to guide users in better diagnosis and repair of Ethernet physical layer problems. With exception of link_up, all statistics have a default value of 0 when not present. The value of the link_up statistic should be assumed to be 1.

The following example gives all the shared link setup. In this case mii is used to filter statistics.

kstat ce:0:mii:link_*

22.3.2. DTrace for Dynamic Instrumentation

DTrace is a comprehensive dynamic tracing facility for examining the behavior of both user programs and the operating system itself. With DTrace, you can collect data at strategic locations in your environment, referred to as probes. DTrace enables you to record such data as stack traces, timestamps, the arguments to a function, or simply counts of how often the probe fires. Because DTrace enables you to insert probes dynamically, you do not need to recompile your code. For more information on DTrace, see the Dynamic Tracing Guide and the DTrace User Guide.