path: root/kexec/kexec.8

.\" Process this file with
.\" groff -man -Tascii kexec.8
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.TH kexec 8 "April 2006" Linux "User Manuals"
.SH NAME
kexec \- directly boot into a new kernel
.SH SYNOPSIS
.B /sbin/kexec
.B [-v (\-\-version)] [-f (\-\-force)] [-x (\-\-no-ifdown)] [-y (\-\-no-sync)] [-l (\-\-load)] [-p (\-\-load-panic)] [-u (\-\-unload)] [-e (\-\-exec)] [-t (\-\-type)]
.BI [\-\-mem\-min= addr ]
.BI [\-\-mem\-max= addr ]

.SH DESCRIPTION
.B kexec
is a system call that enables you to load and boot into another
kernel from the currently running kernel.
.B kexec
performs the function of the boot loader from within the kernel. The
primary difference between a standard system boot and a
.B kexec
boot is that the hardware initialization normally performed by the BIOS
or firmware (depending on architecture) is not performed during a
.B kexec
boot. This has the effect of reducing the time required for a reboot.
.PP
Make sure you have selected
.B CONFIG_KEXEC=y
when configuring the kernel. The
.B CONFIG_KEXEC
option enables the
.B kexec
system call.
.SH USAGE
Using
.B kexec
consists of

.RS
(1) loading the kernel to be rebooted to into memory, and
.RE
.RS
(2) actually rebooting to the pre-loaded kernel.
.RE
.PP
To load a kernel, the syntax is as follows:

.RS
.B kexec
.RI \-l\  kernel-image
.RI "\-\-append=" command\-line\-options
.RI "\-\-initrd=" initrd\-image
.RE

where
.I kernel\-image
is the kernel file that you intend to reboot to.
.PP
Insert the command-line parameters that must be passed to the new
kernel into
.IR command\-line\-options .
Passing the exact contents of /proc/cmdline into
.I command\-line\-options
is the safest way to ensure that correct values are passed to the
rebooting kernel.
.PP
The optional
.I initrd-image
is the initrd image to be used during boot.
.PP
It's also possible to invoke
.B kexec
without an option parameter. In that case, kexec loads the specified
kernel and then invokes
.BR shutdown (8).
If the shutdown scripts of your Linux distribution support
kexec-based rebooting, they then call
.B kexec
.I -e
just before actually rebooting the machine. That way, the machine does
a clean shutdown including all shutdown scripts.

.SH EXAMPLE
.PP
For example, if the kernel image you want to reboot to is
.BR /boot/vmlinux ,
the contents of /proc/cmdline is
.BR "root\=/dev/hda1" ,
and the path to the initrd is
.BR /boot/initrd ,
then you would use the following command to load the kernel:

.RS
.B kexec
.RB \-l\  /boot/vmlinux
.RB "\-\-append=" "root=/dev/hda1" "\ \-\-initrd=" /boot/initrd
.RE
.PP
After this kernel is loaded, assuming the user-space supports kexec-based
rebooting, it can be booted to, with a clean shutdown, using the command:

.RS
.BR reboot
.RE
.PP
Alternatively, it can also be booted to, without calling shutdown(8), with
the command:

.RS
.BR kexec \ \-e
.RE

.SH OPTIONS
.TP
.B \-d\ (\-\-debug)
Enable debugging messages.
.TP
.B \-S\ (\-\-status)
Return 1 if the type (by default crash) is loaded, 0 if not. Can be used in
conjuction with -l or -p to toggle the type. Note this option supersedes other
options and it will
.BR not\ load\ or\ unload\ the\ kernel.
.TP
.B \-e\ (\-\-exec)
Run the currently loaded kernel. Note that it will reboot into the loaded kernel without calling shutdown(8).
.TP
.B \-f\ (\-\-force)
Force an immediate
.B kexec
call, do not call
.BR shutdown (8)
(contrary to the default action without any option parameter). This option
performs the same actions like executing
.IR -l
and
.IR -e
in one call.
.TP
.B \-h\ (\-\-help)
Open a help file for
.BR kexec .
.TP
.B \-\-hotplug
Setup for kernel modification of the elfcorehdr. This option performs
the steps needed to support kernel updates to the elfcorehdr in the
presence of hot un/plug and/or on/offline events. This option only
useful for KEXEC_LOAD syscall.
.TP
.B \-i\ (\-\-no-checks)
Fast reboot, no memory integrity checks.
.TP
.BI \-l\ (\-\-load) \ kernel
Load the specified
.I kernel
into the current kernel.
.TP
.B \-p\ (\-\-load\-panic)
Load the new kernel for use on panic.
.TP
.BI \-t\ (\-\-type= type )
Specify that the new kernel is of this
.I type.
.TP
.BI \-s\ (\-\-kexec-file-syscall)
Specify that the new KEXEC_FILE_LOAD syscall should be used exclusively.
Ignored on XEN.
.TP
.BI \-c\ (\-\-kexec-syscall)
Specify that the old KEXEC_LOAD syscall should be used exclusively.
.TP
.BI \-a\ (\-\-kexec-syscall-auto)
Try the new KEXEC_FILE_LOAD syscall first and when it is not supported or the
kernel does not understand the supplied image fall back to the old KEXEC_LOAD
interface.

There is no one single interface that always works, so this is the default.

KEXEC_FILE_LOAD is required on systems that use locked-down secure boot to
verify the kernel signature.  KEXEC_LOAD may be also disabled in the kernel
configuration.

KEXEC_LOAD is required for some kernel image formats and on architectures that
do not implement KEXEC_FILE_LOAD.
.TP
.B \-u\ (\-\-unload)
Unload the current
.B kexec
target kernel. If a capture kernel is being unloaded then specify -p with -u.
.TP
.B \-v\ (\-\-version)
Return the version number of the installed utility.
.TP
.B \-x\ (\-\-no\-ifdown)
Shut down the running kernel, but restore the interface on reload.
.TP
.B \-y\ (\-\-no\-sync)
Shut down the running kernel, but skip syncing the filesystems.
.TP
.BI \-\-mem\-min= addr
Specify the lowest memory address
.I addr
to load code into.
.TP
.BI \-\-mem\-max= addr
Specify the highest memory address
.I addr
to load code into.
.TP
.BI \-\-entry= addr
Specify the jump back address. (0 means it's not jump back or preserve context)
.TP
.BI \-\-load\-preserve\-context
Load the new kernel and preserve context of current kernel during kexec.
.TP
.BI \-\-load\-jump\-back\-helper
Load a helper image to jump back to original kernel.
.TP
.BI \-\-reuseinitrd
Reuse initrd from first boot.
.TP
.BI \-\-print-ckr-size
Print crash kernel region size, if available.


.SH SUPPORTED KERNEL FILE TYPES AND OPTIONS
.B Beoboot-x86
.RS
.TP
.B \-\-args\-elf
Pass ELF boot notes.
.TP
.B \-\-args\-linux
Pass Linux kernel style options.
.TP
.B \-\-real\-mode
Use the kernel's real mode entry point.
.RE
.PP
.B elf-x86
.RS
.TP
.BI \-\-append= string
Append
.I string
to the kernel command line.
.TP
.BI \-\-command\-line= string
Set the kernel command line to
.IR string .
.TP
.BI \-\-reuse-cmdline
Use the command line from the running system. When a panic kernel is loaded, it
strips the
.I
crashkernel
parameter automatically. The
.I BOOT_IMAGE
parameter is also stripped.
.TP
.BI \-\-initrd= file
Use
.I file
as the kernel's initial ramdisk.
.TP
.BI \-\-ramdisk= file
Use
.I file
as the kernel's initial ramdisk.
.RE
.PP
.B bzImage-x86
.RS
.TP
.BI \-\-append= string
Append
.I string
to the kernel command line.
.TP
.BI \-\-command\-line= string
Set the kernel command line to
.IR string .
.TP
.BI \-\-reuse-cmdline
Use the command line from the running system. When a panic kernel is loaded, it
strips the
.I
crashkernel
parameter automatically. The
.I BOOT_IMAGE
parameter is also stripped.
.TP
.BI \-\-initrd= file
Use
.I file
as the kernel's initial ramdisk.
.TP
.BI \-\-ramdisk= file
Use
.I file
as the kernel's initial ramdisk.
.TP
.BI \-\-real-mode
Use real-mode entry point.
.RE
.PP
.B multiboot-x86
.RS
.TP
.BI \-\-command\-line= string
Set the kernel command line to
.IR string .
.TP
.BI \-\-reuse-cmdline
Use the command line from the running system. When a panic kernel is loaded, it
strips the
.I
crashkernel
parameter automatically. The
.I BOOT_IMAGE
parameter is also stripped.
.TP
.BI \-\-module= "mod arg1 arg2 ..."
Load module
.I mod
with command-line arguments
.I "arg1 arg2 ..."
This parameter can be specified multiple times.
.RE
.PP
.B multiboot2-x86
.RS
.TP
.BI \-\-command\-line= string
Set the kernel command line to
.IR string .
.TP
.BI \-\-reuse-cmdline
Use the command line from the running system. When a panic kernel is loaded, it
strips the
.I
crashkernel
parameter automatically. The
.I BOOT_IMAGE
parameter is also stripped.
.TP
.BI \-\-module= "mod arg1 arg2 ..."
Load module
.I mod
with command-line arguments
.I "arg1 arg2 ..."
This parameter can be specified multiple times.
.RE
.PP
.B elf-ppc64
.RS
.TP
.BI \-\-reuse-cmdline
Use the kernel command line from the running system.
.TP
.BI \-\-command\-line= string
Set the kernel command line to
.IR string.
.TP
.BI \-\-append= string
Set the kernel command line to
.IR string.
.TP
.BI \-\-ramdisk= file
Use
.IR file
as the initial RAM disk.
.TP
.BI \-\-initrd= file
Use
.IR file
as the initial RAM disk.
.TP
.BI \-\-devicetreeblob= file
Specify device tree blob file. Not applicable while using --kexec-file-syscall.
.TP
.BI \-\-dtb= file
Specify device tree blob file. Not applicable while using --kexec-file-syscall.
.RE

.SH ARCHITECTURE OPTIONS
.TP
.B \-\-console\-serial
Enable the serial console.
.TP
.B \-\-console\-vga
Enable the VGA console.
.TP
.B \-\-elf32\-core\-headers
Prepare core headers in ELF32 format.
.TP
.B \-\-elf64\-core\-headers
Prepare core headers in ELF64 format.
.TP
.B \-\-reset\-vga
Attempt to reset a standard VGA device.
.TP
.BI \-\-serial= port
Specify the serial
.I port
for debug output.
.TP
.BI \-\-serial\-baud= baud_rate
Specify the
.I baud rate
of the serial port.
.TP
.BI \-\-dt\-no\-old\-root
Do not reuse old kernel root=<device>
param while creating flatten device tree.