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NetBSD Documentation: Kernel

Frequently Asked Kernel Questions

Frequently Asked Hardware Questions

Further reading

Frequently Asked Kernel Questions

Where to download the kernel source

Official release source

To compile a custom kernel for the same release you already have installed, you only need the syssrc.tgz file. This can be found under the 'source/sets' directory for any given release. For example, NetBSD 7.1.2 kernel sources are located at

If you have a NetBSD CD-ROM, the 'source/syssrc.tgz' file should be included. The source can be extracted anywhere, though the traditional location is inside /usr/src. To extract use "cd / ; tar xvzpf <FILENAME>".

'Bleeding edge' -current source, for the adventurous only!

The latest kernel sources are available from or one of the mirrors in the directory /pub/NetBSD/NetBSD-current/src/sys/. To compile a kernel you should download the following from /pub/NetBSD/NetBSD-current/tar_files/src:

You should first build and install the 'config' program, in case it has changed since the version you are running. Since -current is on the active edge of NetBSD development, there can be problems compiling a -current kernel. You are recommended to use source from an Official Release until you are familiar with the configuration process.

Downloading the kernel source from a certain date

You might need to do this if you have installed a snapshot on your machine and need to rebuild the kernel (and the -current kernel is too recent). Follow the directions on how to Track NetBSD-current with anoncvs.

How to build a kernel

The procedure below applies only if you are compiling the kernel of the same version of NetBSD that you have already installed. Updating to a newer kernel of the same major version should be also fine using this procedure, but if you update a -current kernel, or want to update to a newer major release, compiling a new toolchain first is required. Follow the description of building the toolchain and new kernel with the script in the documentation about tracking -current: The steps to build a kernel are:
  1. Ensure you have installed the Compilers (comp.tgz) set that came with your base system.
  2. Download and extract the kernel source (see Where to download kernel source).
  3. "cd /usr/src/sys/arch/<ARCH>/conf", where <ARCH> is your machine's architecture such as 'i386', 'sparc', 'mac68k'.
  4. "cp GENERIC <MYCONF>", where <MYCONF> is your name for this configuration. You could use your hostname, the machine type, or even your first name. Keep to letters, numbers, and _ characters.
  5. Edit <MYCONF>. Initially you can skip this stage. You can remove drivers for CPU types, hardware, and devices you do not have or use, or even enable options, such as on i386 commenting out the 'pc0' line and enabling the 'vt0' to gain virtual consoles. A good start to determining what hardware drivers you definitely need to keep is to read the output of "dmesg" or "dmesg | grep ' at '". For every line containing '<XXX> at <YYY>' you need to keep the entries for both <XXX> and <YYY>. You should also read options(4) for information on the different kernel configuration options.
  6. run config <MYCONF>, which will generate the kernel build directory for <MYCONF>.
  7. "cd ../compile/<MYCONF>" changes to the kernel build directory.
  8. "make depend" generates a '.depend' file that enables the make program to see what needs to be rebuilt (at this point it will be everything!).
  9. "make" will compile the kernel. If all goes well you will be left with a 'netbsd' kernel. This may take some significant time if you are on a VAX, some time on a big Alpha, and somewhere in-between for the rest of us.
  10. "mv /netbsd /netbsd.old ; mv /usr/src/sys/arch/<ARCH>/compile/<MYCONF>/netbsd /" saves your current kernel, (very important), and moves the new kernel ready to be booted.
  11. "reboot" should reboot using your new kernel - the boot messages should contain a line of the form: 'NetBSD <VERSION> (<MYCONF>) #0: <COMPILE_DATE>'
  12. If you have any problems: You should boot your 'netbsd.old' kernel in single user mode. The procedure varies from port to port depending on the boot procedure, but on i386 it would be:
    1. Press SPACE when the first NetBSD message appears
    2. "boot netbsd.old -s"
    Then swap your kernel back:
    1. "fsck /"
    2. "mount /"
    3. "mv netbsd.old netbsd"
    4. "exit"

What exactly is a GENERIC kernel?

The term GENERIC refers to a kernel that is configured to run on just about any machine supported by the machine architecture. The term originated from a line in the kernel configuration file which specified that the root device was "generic" as well as a configuration option. This option and that format of the configuration line is no longer used, but the name will probably stick for a while.

Since these kernels tend to include support for all the available device drivers and many models of machines that you are not using, you are encouraged to compile your own custom kernel.

What does mclpool limit reached: increase NMBCLUSTERS mean?

This means the kernel has run out of space to map mbuf clusters. mbuf clusters are used by the network code to store packets and other network related data.

The default setting for NMBCLUSTERS is 1024 (256 in NetBSD 1.5 and earlier), so if you have this problem, try doubling the value until the error message disappears. To display the current value of NMBCLUSTERS you can use sysctl(8) as follows:

	# sysctl kern.mbuf.nmbclusters

Alternatively, try

        # echo 'print nmbclusters' | gdb -q /netbsd

See also options(4) for more details on kernel configuration options. To change the value, add

	options NMBCLUSTERS=2048

to your kernel configuration, or patch the binary:

        # gdb --write /netbsd
        (gdb) set nmbclusters=2048
        (gdb) quit

Note that if you patch the binary only, you will need to reboot for the change to take effect. If you're on a platform which supports it, you can set the value with:

	# sysctl -w kern.mbuf.nmbclusters=2048

This will work, but will be lost on the next reboot. Combining this, and patching the binary, would mean no need to build a new kernel or reboot.


This kernel message means that there is a bug in the kernel where a syscall did "int x = splfoo();" and did not call "splx(x);" before it returned. The splx(x); function in this example would restore the system priority level to the one encoded in x, which was a value previously returned by one of the other spl functions (in this case, the made up example of splfoo();).

If you get this kernel message you should be dropped into ddb(4), the in-kernel debugger. A stack trace in ddb, accomplished by pushing 't', might show you the offending syscall(). It is probably a good idea to send-pr(1) the output of the trace command (as well as any other relevant information), since you should under no circumstances be getting this kernel message.

See also spl(9) for more information on spl functions.

What does Stray interrupt on IRQ 7 mean?

The "Stray interrupt on IRQ 7" kernel message means that the interrupt controller reported an unmasked interrupt on IRQ 7, but no driver attached to that IRQ 'claimed' it.

There are two reasons this can happen:

  1. In anything other than a PC, it would almost always means that there is a driver attached to the IRQ (otherwise it would be masked), but it is the wrong driver.

  2. In a PC, there is the more obscure issue of 'default IR7's. That is, when a device asserts an IRQ, but the IRQ is deasserted after the PIC latches the interrupt and before the CPU acknowledges it, the PIC just flat out lies about which IRQ it was.

    There is a scheme for recognizing 'default IR7's, but it turns out that it fails badly on some older systems, and in general it's better to fix drivers to not generate them in the first place. In some cases it's difficult to completely prevent them when using edge-triggered interrupts though.

You should only get this kernel message running a kernel with the DEBUG option defined.

Why are kernels compiled with -msoft-float

When a process makes a system call the kernel needs to save the processor state, so that it can later switch back to the process. Floating point registers tend to be large and relatively plentiful, making saving and restoring them an expensive operation. If the FPU is in the middle of an operation the CPU will additionally be forced to sit and wait for it to finish before it can then copy the registers.

Avoiding floating point registers in the kernel gives a significant performance win for system calls. Some processors, such as sparc, can also use lazy FP context switching to sometimes avoid having to save and restore FP registers even when switching between processes.

On some architectures the compiler can use floating point registers to speed up certain operations (such as block memory copies), breaking the above, so '-msoft-float' is required.

Kernel compiles on low memory machine seem very slow

By default NetBSD installs a GENERIC kernel which includes drivers for almost every supported item of hardware, network protocol, and filesystem. While this allows one kernel to run on virtually every machine for a given port, it does result in it using more space than is really needed, particularly on a small memory machine. This is compounded by kernel compiles using the -O2 optimisation level which tells the compiler to use extra memory and time to make the kernel as fast as possible.

One option when building your own kernel is to use "make COPTS=-O" which instructs the compiler to perform only the most effective optimisations. This will result in a fractionally slower kernel, but it will take less time to compile.

If you are intending to take several 'compile and reboot into new kernel' passes while customising a kernel on a low memory machine it may make sense to make the first few passes using "make COPTS=-O", and then switch to "make" for the final pass.

Of course, generally the fastest way to compile a kernel on a low memory machine is to use another machine, or temporarily add some more memory!

Problems compiling a -current kernel

The first point to note is you should subscribe to the current-users mail list. Tracking -current without reading current-users is akin to driving in the dark without any lights. You have been warned :).

It is always worth downloading the latest config.tar.gz, compiling, installing and rerunning on your config file - config changes reasonably frequently between releases.

Sometimes, binaries and/or libraries need to be updated before you will be able to build -current on a release. In these situations, it may be simpler to install from a binary snapshot, and then build -current. Snapshots of -current for i386 (for example) can be found in /pub/NetBSD/arch/i386/snapshot/. The src/UPDATING file contains information about these important changes which you should be aware of when attempting to build -current, or a -current kernel.

Debugging a kernel crash dump

  1. Ensure you have built a kernel from the same source with DEBUG, and 'makeoptions DEBUG="-g"' enabled in the config file.
  2. "gdb netbsd.gdb" (in kernel compile directory).
  3. "target kvm /var/crash/netbsd.0.core" at the gdb prompt. Use "target kcore ..." if your system has gdb5 instead of gdb6.

You can use the usual gdb(1) commands, such as 'bt' to get a backtrace.

Getting backtraces when debugging a kernel crash dump

You can get backtraces of an arbitrary process from gdb when debugging a kernel crash dump with two easy steps:

  1. get the address of the lwp structure of the LWP : ps -ax -O laddr -M netbsd.x.core

    (LWP or lightweight process correspond to a process, or a thread of a process which runs in the kernel. For nonthreaded programs, there is exactly one LWP corresponding to the process, for threaded there can be more of them.)

  2. tell gdb to use it "kvm proc 0x<addr>"

What is DDB and what can I do in it?

DDB is the optional in-kernel debugger. It is usually entered via one of three methods:

  • At any time via a port specific key sequence (see ddb(4) for a list).
  • It can be set to be invoked when the kernel panics.
  • By specifying '-d' as a boot flag (boot netbsd -d).

Some of the more useful commands are:

  • trace - Produce a stack trace. Very useful when submitting a PR on a kernel panic.
  • reboot - Reboot the system.
  • sync - produce a crashdump and reboot

Generating a kernel crash dump

Usually the kernel will automatically generate a crashdump on panic, which is then picked up by savecore(8) on reboot. However you can force a crashdump in ddb(4) by using sync (or reboot 0x100). If the kernel panics or hangs while attempting to sync the buffer cache you can use reboot 0x104 which will bypass the sync.

Adding a kernel to a boot floppy

Some ports are already setup to build a boot floppy by "cd /usr/src/distrib/<ARCH>/floppies ; make ". (You may need to build the INSTALL kernel manually before running this. If you have an existing boot.fs image you can replace the kernel with:

  1. vnconfig -c vnd0 boot.fs
  2. mount /dev/vnd0a /mnt
  3. gzip -c -9 < netbsd > /mnt/netbsd.gz
  4. umount /mnt
  5. vnconfig -u vnd0

This assumes you have "pseudo-device vnd" in your kernel config file, and a ready to use kernel.

I'm having trouble mounting the SCSI device for a new partition I've just created. What's up with this device-numbering scheme?

By default, SCSI devices under NetBSD are numbered starting from 0 in the order of SCSI ID number. In other words, you lowest-numbered SCSI device will be /dev/sd0, the next device will be /dev/sd1, etc. Notice that this is the assignment that they are given during the boot process.

If you compile your own kernels, you can set the SCSI devices to point to any SCSI ID number you want with a kernel configuration line like:

sd0             at scsibus0 target 4 lun 0
sd*             at scsibus? target ? lun ?

The above lines will make device sd0 point to the disk at SCSI ID#4 and the rest of the devices will be assigned as described above. This is often referred to as "hardwiring" your SCSI devices, and is recommended if you are using RAIDframe or ccd so as to avoid the device IDs being changed out from under the configuration if one device is powered off or broken.

Frequently Asked Hardware Questions

What does device not configured mean?

  • If this message appears during the autoconfiguration output of system boot, it means that the kernel discovered a piece of hardware in your system that it doesn't have a device driver for. This means that either the device driver exists and has not been compiled into the kernel you booted, or the device driver doesn't exist at all (in which case, it's time to contact a friendly developer and offer him testing hardware in exchange for writing a device driver).

    Since GENERIC kernels are used for basic installation, it is important that they be stable and known to work; as such, device drivers that are not yet stable are not compiled into GENERIC kernels by default. Examination of a GENERIC kernel configuration file for your system may reveal experimental device drivers for your device which are "commented out." If you compile a kernel of your own (please don't call it GENERIC), you can try experimental device drivers.

  • If this message appears when you try to access a device node in /dev (e.g. a SCSI disk), this means that the driver can't find the specific device unit you tried to access, e.g. accessing a SCSI disk that isn't there, or the driver is not compiled in the kernel.

    Often, this happens when the device nodes in /etc/fstab don't match what the kernel found during autoconfiguration at boot time, and the "mount" command in /etc/rc tries to mount all the filesystems. You should double check that the devices you're trying to use were actually found by the kernel at boot time, by examining /var/run/dmesg.boot (a saved copy of the boot time autoconfiguration output).

    Another case where this can happen is when a certain kernel subsystem which is implemented as a pseudo-device is not compiled into the kernel or loaded as an LKM and a configuration program wants to configure it using a device node in /dev. For example when a firewall is not compiled into the kernel or loaded as an LKM and the pfctl(8) or ipf(8) utility attempts to load firewall rules. If such utility does not print a helpful message indicating what device it tried to use, ktrace(1) can help you find what's going on inside a command, and to determine you what's being accessed that may cause the error message.

    And it can happen in many other cases when a nonexistent device or device without a driver is accessed, like when a nonexistent network interface name is passed to ifconfig(8) (in this case, if you are sure that you have the right driver, maybe the interface must be explicitly created by a command like ifconfig vlan0 create — this is true for most of the network pseudo-devices like sl(4), vlan(4) or stf(4)).

Debugging ATAPI or ATA (IDE) devices

If a kernel is compiled with WDCDEBUG defined, then gdb can be used to patch wdcdebug_atapi_mask and wdcdebug_mask. Setting the appropriate bits in these variables will cause the kernel to output verbose information about ATAPI and ATA operations. (Currently NetBSD defaults to WDCDEBUG enabled.)

For the maximum level of output, use:

	# gdb --write /netbsd
	(gdb) set wdcdebug_atapi_mask=0xff
	(gdb) set wdcdebug_mask=0xff
	(gdb) quit

Note: This will produce an extremely large quantity of output. To select individual options, look for the list of bitflags directly above:

Debugging USB devices

If you have problems with USB devices, you can enable verbose messages in the usb driver:

  • Compile a kernel with USB_DEBUG and DDB defined.
  • Boot with -d to enter ddb(4).
  • In ddb set the variables usbdebug and uhcidebug to 5 ("write usbdebug 5" and "write uhcidebug 5")
  • Plug something in and type continue.

USB debugging code has been changed in NetBSD 7.99 and later. The changes mean that the core and host controller debug output can be controlled by sysctl(8) and read using vmstat(1).

Device drivers are being converted to this new method.

  • Compile a kernel with USB_DEBUG and/or [EOUX]HCI_DEBUG defined. Each device driver has its own define. The host controller and device defines implicitly define USB_DEBUG.
  • Boot the kernel and using sysctl(8) set the hw.usb.debug variable to a non-zero value (higher values give more output). Other variables that can be set are hw.[eoux]hci.debug.
  • Plug something in and use vmstat -u usbhist to dump the debug output.

Recognising a new PnP device

This assumes the device is of a generic type which is already supported, but the device ID is not recognised. Adding a device that performs differently includes writing code.

  1. It should be reported in the boot messages as 'not configured'. Note the device ID (in this case USR3031):

    isapnp0: <U.S. Robotics 56K FAX INT, USR3031, , > port 0x3e8/8 irq 5 not configured
  2. Add an appropriate entry to /usr/src/sys/dev/isapnp/isapnpdevs:

    devlogic       com     USR3031         USR 56k Faxmodem
  3. Regenerate isapnpdevs.{c,h} using 'make -f Makefile.isapnpdevs'.
  4. Rebuild the kernel
  5. Submit a PR with the changes, via send-pr(1) or the online form.

Recognising a new PCMCIA device

This assumes the device is of a generic type which is already supported, but the device ID is not recognised. Adding a device that performs differently includes writing code.

  1. Compile a kernel with options(4) PCMCIAVERBOSE.
  2. Check the boot messages - the card should be reported as 'not configured'. Note the Manufacturer and product codes (in this case 0x143 and 0x201):
    pcmcia0: CIS version PCMCIA 2.0 or 2.1
    pcmcia0: CIS info: Grey Cell, GCS2000, Gold II, 1
    pcmcia0: Manufacturer code 0x143, product 0x201
    pcmcia0: function 0: network adapter, ccr addr 3f8 mask 1
  3. Add vendor and product entries to /usr/src/sys/dev/pcmcia/pcmciadevs
  4. Regenerate pcmciadevs.h and pcmciadevs_data.h using 'make -f Makefile.pcmciadevs'.
  5. Add an entry to the device table at the top of the appropriate bus attach file in /usr/src/sys/dev/pcmcia/, for example an ne2000 compatible card would use /usr/src/sys/dev/pcmcia/if_ne_pcmcia.c
  6. Rebuild the kernel
  7. Submit a PR with the changes, via send-pr(1) or the online form.

What is UBC?

UBC stands for the Unified Buffer Cache project. It was written by Chuck Silvers, and has been integrated into NetBSD since 1.5L (Nov 2000). When upgrading from a non-UBC setup, you'll need to rerun config(8) again, but before you do, you'll want to remove any settings for "BUFCACHE", "NBUF" or "BUFPAGES", and let the size of the buffer cache go back to the default. Under UBC, the traditional buffer cache is no longer used for storing regular file data, only metadata, so you want to allow the VM system to manage most of your physical memory. The default buffer cache size will be fine for most people, regardless of the amount of memory in the machine. The amount of memory in the boot messages about "using X buffers containing Y memory" no longer indicates the amount of memory available for caching file data, so don't worry if those numbers don't change.

The important difference is that more memory will be available for caching regular file data, so filesystem i/o will be faster since there will be more times when the data you're accessing is already in memory. How much faster depends on what you're doing, but you'll probably notice the difference.

See also: UBC: An Efficient Unified I/O and Memory Caching Subsystem for NetBSD by Chuck Silvers.

Further reading

NetBSD specific documentation

Other online documentation

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