Documentation/s390/zfcpdump.txt - kernel/bruno - Git at Google

 The s390 SCSI dump tool (zfcpdump)

 System z machines (z900 or higher) provide hardware support for creating system
 dumps on SCSI disks. The dump process is initiated by booting a dump tool, which
 has to create a dump of the current (probably crashed) Linux image. In order to
 not overwrite memory of the crashed Linux with data of the dump tool, the
 hardware saves some memory plus the register sets of the boot CPU before the
 dump tool is loaded. There exists an SCLP hardware interface to obtain the saved
 memory afterwards. Currently 32 MB are saved.

 This zfcpdump implementation consists of a Linux dump kernel together with
 a user space dump tool, which are loaded together into the saved memory region
 below 32 MB. zfcpdump is installed on a SCSI disk using zipl (as contained in
 the s390-tools package) to make the device bootable. The operator of a Linux
 system can then trigger a SCSI dump by booting the SCSI disk, where zfcpdump
 resides on.

 The kernel part of zfcpdump is implemented as a debugfs file under "zcore/mem",
 which exports memory and registers of the crashed Linux in an s390
 standalone dump format. It can be used in the same way as e.g. /dev/mem. The
 dump format defines a 4K header followed by plain uncompressed memory. The
 register sets are stored in the prefix pages of the respective CPUs. To build a
 dump enabled kernel with the zcore driver, the kernel config option
 CONFIG_CRASH_DUMP has to be set. When reading from "zcore/mem", the part of
 memory, which has been saved by hardware is read by the driver via the SCLP
 hardware interface. The second part is just copied from the non overwritten real
 memory.

 Since kernel version 3.12 also the /proc/vmcore file can also be used to access
 the dump.

 To get a valid zfcpdump kernel configuration use "make zfcpdump_defconfig".

 The s390 zipl tool looks for the zfcpdump kernel and optional initrd/initramfs
 under the following locations:

 * kernel:  <zfcpdump directory>/zfcpdump.image
 * ramdisk: <zfcpdump directory>/zfcpdump.rd

 The zfcpdump directory is defined in the s390-tools package.

 The user space application of zfcpdump can reside in an intitramfs or an
 initrd. It can also be included in a built-in kernel initramfs. The application
 reads from /proc/vmcore or zcore/mem and writes the system dump to a SCSI disk.

 The s390-tools package version 1.24.0 and above builds an external zfcpdump
 initramfs with a user space application that writes the dump to a SCSI
 partition.

 For more information on how to use zfcpdump refer to the s390 'Using the Dump
 Tools book', which is available from
 http://www.ibm.com/developerworks/linux/linux390.
	The s390 SCSI dump tool (zfcpdump)

	System z machines (z900 or higher) provide hardware support for creating system
	dumps on SCSI disks. The dump process is initiated by booting a dump tool, which
	has to create a dump of the current (probably crashed) Linux image. In order to
	not overwrite memory of the crashed Linux with data of the dump tool, the
	hardware saves some memory plus the register sets of the boot CPU before the
	dump tool is loaded. There exists an SCLP hardware interface to obtain the saved
	memory afterwards. Currently 32 MB are saved.

	This zfcpdump implementation consists of a Linux dump kernel together with
	a user space dump tool, which are loaded together into the saved memory region
	below 32 MB. zfcpdump is installed on a SCSI disk using zipl (as contained in
	the s390-tools package) to make the device bootable. The operator of a Linux
	system can then trigger a SCSI dump by booting the SCSI disk, where zfcpdump
	resides on.

	The kernel part of zfcpdump is implemented as a debugfs file under "zcore/mem",
	which exports memory and registers of the crashed Linux in an s390
	standalone dump format. It can be used in the same way as e.g. /dev/mem. The
	dump format defines a 4K header followed by plain uncompressed memory. The
	register sets are stored in the prefix pages of the respective CPUs. To build a
	dump enabled kernel with the zcore driver, the kernel config option
	CONFIG_CRASH_DUMP has to be set. When reading from "zcore/mem", the part of
	memory, which has been saved by hardware is read by the driver via the SCLP
	hardware interface. The second part is just copied from the non overwritten real
	memory.

	Since kernel version 3.12 also the /proc/vmcore file can also be used to access
	the dump.

	To get a valid zfcpdump kernel configuration use "make zfcpdump_defconfig".

	The s390 zipl tool looks for the zfcpdump kernel and optional initrd/initramfs
	under the following locations:

	* kernel: <zfcpdump directory>/zfcpdump.image
	* ramdisk: <zfcpdump directory>/zfcpdump.rd

	The zfcpdump directory is defined in the s390-tools package.

	The user space application of zfcpdump can reside in an intitramfs or an
	initrd. It can also be included in a built-in kernel initramfs. The application
	reads from /proc/vmcore or zcore/mem and writes the system dump to a SCSI disk.

	The s390-tools package version 1.24.0 and above builds an external zfcpdump
	initramfs with a user space application that writes the dump to a SCSI
	partition.

	For more information on how to use zfcpdump refer to the s390 'Using the Dump
	Tools book', which is available from
	http://www.ibm.com/developerworks/linux/linux390.