VM Shared Kernel Support

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Note: 11 May 2009
The VM Share Kernel Support procedure documented on this page was created a few years ago for use with an older Linux Kernel.

  • Linux kernel 2.6.21 (including the Development stream) and later provides NSS support.
  • Linux kernel 2.6.16 (October 2005 stream) provides NSS support in the kernel patches.

For documentation about NSS, see "Device Drivers, Features, and Commands" book on the developerWorks documentation page. The 'Shared kernel support' chapter describes how you can create and maintain a Linux NSS.

If your Linux distribution supports the "VM shared kernel support" configuration option, the Linux kernel can be generated as a shareable NSS (named saved system). Once generated with this option, the system must be IPLed, stopped at location x'10000' and saved (from a class E user) with something like the following example from a 64M machine:


Once this is done, any VM user can IPL LXSHR and about 1.5M of the kernel is shared among all users. Obviously, the more Linux virtual machines running, the greater the benefit of using the shared system.

Every virtual machine that IPLs your shared system must have the same disk configuration as the system that was saved. That is, the disks must be at the same addresses and be the same sizes as the virtual machine that issued the SAVESYS command.

An example of how to use VM Shared Kernel Support

Below is a sample EXEC that can be used to help you create a shared Linux NSS. Once you have generated a kernel with the "VM shared kernel support" config option and have that kernel ready to IPL from a device, the EXEC can be used as follows:

Userid Class
The VM userid that will run the EXEC should be a class E VM userid.

Storage size
The userid virtual machine storage should be defined to whatever size you want as the minimum size of the virtual machine to IPL your shared system. At least 64M is recommended.

Invoking the SAVELX EXEC

   SAVELX system_name device_number

is the name you want to give to your shared system (e.g. lxshr)
is the device number that you usually IPL to start your Linux system.
CP may take a couple minutes to save your system. When the SAVELX EXEC is complete, the VM userid will be in CP READ mode and at that point you or another VM user will be able to start up your shared system. by issuing the command:

I system_name


   /* SAVELX EXEC */

/* get the system name and device to ipl */
parse arg lxname devnum
lxname = strip(lxname)
devnum = strip(devnum)

/* figure out the line end character */
'pipe cp q term | var termout'
parse var termout one myend three
myend = strip(myend)

/* figure out the storage size */
'pipe cp q v stor | var storout'
parse var storout one two storsize

/* construct the defsys command */
DODEF = 'DEFSYS' lxname '0-FF EW 100-2FF SR 300-4FF EW'
dodef = dodef 'MINSIZE=' || storsize
say dodef
/* define the saved system */
/* arrange to stop the ipl processing at the appropriate spot, */
/* at which point a savesys will be issued */
setsave = setsave || myend 'TRACE END ALL'
say setsave

doipl = 'i' devnum
say doipl

/* all set, issue the ipl */

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