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Simon Crowe
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kexec: Provide user friendly option for memory limit grub2 requires that the '<' character be escaped which is inconvienet for users, provide a more natural specifier. Signed-off-by: Simon Crowe <Simon.Crowe@citrix.com> Reviewed-by: Jan Beulich <jbeulich@suse.com> Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
217 lines
7.1 KiB
Plaintext
217 lines
7.1 KiB
Plaintext
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=======================
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Kexec and Kdump for Xen
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=======================
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This is a brief guide to using Kexec and Kdump in conjunction with Xen.
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This functionally works at the level of the hypervisor and dom0 kernel.
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And will thus affect all guests running on a machine.
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At this stage it does not work in conjunction with domU kernels.
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This document should be read in conjunction with
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Documentation/kdump/kdump.txt from the Linux kernel source.
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Some of the information in this document has been
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sourced from that document.
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Kexec
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=====
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It is possible to kexec from Xen or Linux to either Xen or Linux.
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Pattern | Before Kexec | After Kexec
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---------------+--------------------+--------------------
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Xen -> Xen | first hypervisor & | second hypervisor &
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| dom0 kernel | dom0 kernel
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---------------+--------------------+--------------------
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Xen -> Linux | first hypervisor & | second kernel
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| dom0 kernel |
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---------------+--------------------+--------------------
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Linux -> Xen | first kernel | second hypervisor &
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| | dom0 kernel
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---------------+--------------------+--------------------
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Linux -> Linux | first kernel | second kernel
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If you are kexecing to Xen then you will also need to prepare the second
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hypervisor and dom0 kernel that will run after kexec. These may be the same
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as the first hypervisor and dom0 kernel that are used before kexec if you
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are kexecing from Xen to Xen.
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If you are kexecing to Linux then you will need to prepare the second Linux
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kernel that will run after kexec. In the case that you are kexecing from
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Linux, it may be the same as the first kernel image that that runs before
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kexec.
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Regardless of which kexec pattern you wish to run, you will
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need to have kexec-tools installed. This provides the kexec command.
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1. Load
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-------
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Before kexecing the second kernel or hypervisor & dom0 kernel
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need to be loaded into the running hypervisor or kernel using
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the kexec command.
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a. To kexec to Xen (Xen->Xen or Linux->Xen)
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kexec -l --append="XEN_ARGS -- DOM0_ARGS" \
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--vmm="XEN_IMAGE" "DOM0_IMAGE" KEXEC_ARGS
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where:
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XEN_ARGS: command line arguments to the xen hypervisor
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On x86 the no-real-mode argument should be included
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DOM0_ARGS: command line arguments to the dom0 kernel
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XEN_IMAGE: xen hypervisor image
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DOM0_IMAGE: dom0 kernel image
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KEXEC_ARGS: additional kexec-tools command line arguments
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e.g. kexec -l --append "no-real-mode" --vmm="/boot/xen.gz" /boot/vmlinuz.gz
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OR
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b. To kexec to Linux (Xen->Linux or Linux->Linux)
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kexec -l LINUX_IMAGE --append "$LINUX_ARGS" KEXEC_ARGS
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where:
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LINUX_IMAGE: the second linux kernel image
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LINUX_ARGS: command line arguments to the second linux kernel
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KEXEC_ARGS: additional kexec-tools command line arguments
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e.g. kexec -l /boot/second-vmlinuz.gz
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2. Execute
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----------
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Once the second kernel is loaded, it can be executed at any time.
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If you don't see the second kernel booting within a second or so,
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you are in trouble :(
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kexec -e
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Kdump
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=====
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It is possible to kdump from Xen or Linux to a Linux crash kernel.
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It is not possible to use xen as a crash kernel.
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Pattern | Before Kexec | After Kexec
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---------------+--------------------+--------------------
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Xen -> Linux | first hypervisor & | crash kernel
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| dom0 kernel |
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---------------+--------------------+--------------------
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Linux -> Linux | first kernel | crash kernel
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Regardless of if you are kdumping from Xen or Linux you will need to
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prepare a linux crash kernel. You will also need to have kexec-tools
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installed. This provides the kexec command.
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0. Set-Up The Crash Kernel Region
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---------------------------------
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In order to use kdump an area of memory has to be reserved at boot time.
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This is the area of memory that the crash kernel will use, thus allowing it
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to run without disrupting the memory used by the first kernel. This area is
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called the crash kernel region and is reserved using the crashkernel
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command line parameter to the Xen hypervisor. It has two forms:
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i) crashkernel=size
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This is the simplest and recommended way to reserve the crash kernel
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region. Just specify how large the region should be and the hypervisor
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will find a good location for it. A good size to start with is 128Mb
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e.g.
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crashkernel=128M
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ii) crashkernel=size@base
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In this form the base address is provided in addition to
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the size. Use this if auto-placement doesn't work for some reason.
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It is strongly recommended that the base address be aligned
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to 64Mb, else memory below the alignment point will not
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be usable.
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e.g. crashkernel=128M@256M
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iii) crashkernel=size,below=offset
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This allows us to place the crash kernel within the usuable address
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space without having to worry about a specific phyiscal address.
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The '<' and 'below' options are synonymous
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Regardless of which of the forms of the crashkernel command line you
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use, the crash kernel region should appear in /proc/iomem on x86. If it
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doesn't then either the crashkernel parameter is missing, or for some
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reason the region couldn't be placed - for instance because it is too large.
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# cat /proc/iomem
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...
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00100000-07feffff : System RAM
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00100000-00bfffff : Hypervisor code and data
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0533f000-0733efff : Crash kernel
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...
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1. Load
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-------
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Once you are running in a kexec-enabled hypervisor and dom0,
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you can prepare to kdump by loading the crash kernel into the
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running kernel.
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kexec -p CRASH_KERNEL_IMAGE --append "$CRASH_KERNEL_ARGS" KEXEC_ARGS
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where:
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CRASH_KERNEL_IMAGE: the crash kernel image
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CRASH_KERNEL_ARGS: command line arguments to the crash kernel
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init 1 is strongly recommended
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irqpoll is strongly recommended
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maxcpus=1 is required if the crash kernel is SMP
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reset_devices is strongly recommended
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KEXEC_ARGS: additional kexec-tools command line arguments
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On x86 --args-linux should be supplied if an uncompressed
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vmlinux image is used as the crash kernel
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e.g. kexec -p /boot/crash-vmlinuz \
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--append "init 1 irqpoll maxcpus=1 reset_devices" --args-linux
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On x86 systems the crash kernel may be either
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- A uncompressed vmlinux image if the kernel is not relocatable
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- A compressed bzImage or vmlinuz image if the kernel is relocatable
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- Relocatability is controlled by the CONFIG_RELOCATABLE kernel
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compile configuration parameter. This option may not be available
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depending on the kernel version
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2. Execute
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----------
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Once the second kernel is loaded, the crash kernel will be executed if the
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hypervisor panics. It will also be executed if dom0 panics or if dom0
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oopses and /proc/sys/kernel/panic_on_oops is set to a non-zero value
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echo 1 > /proc/sys/kernel/panic_on_oops
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Kdump may also be triggered (for testing)
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a. From Domain 0
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echo c > /proc/sysrq-trigger
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b. From Xen
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Enter the xen console
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ctrl^a ctrl^a (may be bound to a different key, this is the default)
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Select C for "trigger a crashdump"
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C
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If you don't see the crash kernel booting within a second or so,
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you are in trouble :(
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