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=======================
Kexec and Kdump for Xen
=======================
This is a brief guide to using Kexec and Kdump in conjunction with Xen.
This functionally works at the level of the hypervisor and dom0 kernel.
And will thus affect all guests running on a machine.
At this stage it does not work in conjunction with domU kernels.
This document should be read in conjunction with
Documentation/kdump/kdump.txt from the Linux kernel source.
Some of the information in this document has been
sourced from that document.
Kexec
=====
It is possible to kexec from Xen or Linux to either Xen or Linux.
Pattern | Before Kexec | After Kexec
---------------+--------------------+--------------------
Xen -> Xen | first hypervisor & | second hypervisor &
| dom0 kernel | dom0 kernel
---------------+--------------------+--------------------
Xen -> Linux | first hypervisor & | second kernel
| dom0 kernel |
---------------+--------------------+--------------------
Linux -> Xen | first kernel | second hypervisor &
| | dom0 kernel
---------------+--------------------+--------------------
Linux -> Linux | first kernel | second kernel
If you are kexecing to Xen then you will also need to prepare the second
hypervisor and dom0 kernel that will run after kexec. These may be the same
as the first hypervisor and dom0 kernel that are used before kexec if you
are kexecing from Xen to Xen.
If you are kexecing to Linux then you will need to prepare the second Linux
kernel that will run after kexec. In the case that you are kexecing from
Linux, it may be the same as the first kernel image that that runs before
kexec.
Regardless of which kexec pattern you wish to run, you will
need to have kexec-tools installed. This provides the kexec command.
1. Load
-------
Before kexecing the second kernel or hypervisor & dom0 kernel
need to be loaded into the running hypervisor or kernel using
the kexec command.
a. To kexec to Xen (Xen->Xen or Linux->Xen)
kexec -l --append="XEN_ARGS -- DOM0_ARGS" \
--vmm="XEN_IMAGE" "DOM0_IMAGE" KEXEC_ARGS
where:
XEN_ARGS: command line arguments to the xen hypervisor
On x86 the no-real-mode argument should be included
DOM0_ARGS: command line arguments to the dom0 kernel
XEN_IMAGE: xen hypervisor image
DOM0_IMAGE: dom0 kernel image
KEXEC_ARGS: additional kexec-tools command line arguments
e.g. kexec -l --append "no-real-mode" --vmm="/boot/xen.gz" /boot/vmlinuz.gz
OR
b. To kexec to Linux (Xen->Linux or Linux->Linux)
kexec -l LINUX_IMAGE --append "$LINUX_ARGS" KEXEC_ARGS
where:
LINUX_IMAGE: the second linux kernel image
LINUX_ARGS: command line arguments to the second linux kernel
KEXEC_ARGS: additional kexec-tools command line arguments
e.g. kexec -l /boot/second-vmlinuz.gz
2. Execute
----------
Once the second kernel is loaded, it can be executed at any time.
If you don't see the second kernel booting within a second or so,
you are in trouble :(
kexec -e
Kdump
=====
It is possible to kdump from Xen or Linux to a Linux crash kernel.
It is not possible to use xen as a crash kernel.
Pattern | Before Kexec | After Kexec
---------------+--------------------+--------------------
Xen -> Linux | first hypervisor & | crash kernel
| dom0 kernel |
---------------+--------------------+--------------------
Linux -> Linux | first kernel | crash kernel
Regardless of if you are kdumping from Xen or Linux you will need to
prepare a linux crash kernel. You will also need to have kexec-tools
installed. This provides the kexec command.
0. Set-Up The Crash Kernel Region
---------------------------------
In order to use kdump an area of memory has to be reserved at boot time.
This is the area of memory that the crash kernel will use, thus allowing it
to run without disrupting the memory used by the first kernel. This area is
called the crash kernel region and is reserved using the crashkernel
command line parameter to the Xen hypervisor. It has two forms:
i) crashkernel=size
This is the simplest and recommended way to reserve the crash kernel
region. Just specify how large the region should be and the hypervisor
will find a good location for it. A good size to start with is 128Mb
e.g.
crashkernel=128M
ii) crashkernel=size@base
In this form the base address is provided in addition to
the size. Use this if auto-placement doesn't work for some reason.
It is strongly recommended that the base address be aligned
to 64Mb, else memory below the alignment point will not
be usable.
e.g. crashkernel=128M@256M
Regardless of which of the two forms of the crashkernel command line you
use, the crash kernel region should appear in /proc/iomem on x86 or
/proc/iomem_machine on ia64. If it doesn't then either the crashkernel
parameter is missing, or for some reason the region couldn't be placed -
for instance because it is too large.
# cat /proc/iomem
...
00100000-07feffff : System RAM
00100000-00bfffff : Hypervisor code and data
0533f000-0733efff : Crash kernel
...
1. Load
-------
Once you are running in a kexec-enabled hypervisor and dom0,
you can prepare to kdump by loading the crash kernel into the
running kernel.
kexec -p CRASH_KERNEL_IMAGE --append "$CRASH_KERNEL_ARGS" KEXEC_ARGS
where:
CRASH_KERNEL_IMAGE: the crash kernel image
CRASH_KERNEL_ARGS: command line arguments to the crash kernel
init 1 is strongly recommended
irqpoll is strongly recommended
maxcpus=1 is required if the crash kernel is SMP
reset_devices is strongly recommended
KEXEC_ARGS: additional kexec-tools command line arguments
On x86 --args-linux should be supplied if an uncompressed
vmlinux image is used as the crash kernel
e.g. kexec -p /boot/crash-vmlinuz \
--append "init 1 irqpoll maxcpus=1 reset_devices" --args-linux
On x86 systems the crash kernel may be either
- A uncompressed vmlinux image if the kernel is not relocatable
- A compressed bzImage or vmlinuz image if the kernel is relocatable
- Relocatability is controlled by the CONFIG_RELOCATABLE kernel
compile configuration parameter. This option may not be available
depending on the kernel version
On ia64
Either a vmlinuz or vmlinux.gz image may be used
2. Execute
----------
Once the second kernel is loaded, the crash kernel will be executed if the
hypervisor panics. It will also be executed if dom0 panics or if dom0
oopses and /proc/sys/kernel/panic_on_oops is set to a non-zero value
echo 1 > /proc/sys/kernel/panic_on_oops
Kdump may also be triggered (for testing)
a. From Domain 0
echo c > /proc/sysrq-trigger
b. From Xen
Enter the xen console
ctrl^a ctrl^a (may be bound to a different key, this is the default)
Select C for "trigger a crashdump"
C
If you don't see the crash kernel booting within a second or so,
you are in trouble :(
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