Kernel XIP

This page describes the use of Kernel Execute-In-Place as a bootup time reduction technique.

Description
When the kernel is executed in place, the bootloader does not have to:
 * 1) read the kernel from flash or
 * 2) decompress the kernel and
 * 3) write the kernel to RAM.

How to implement or use
TODO: describe how to achieve the technique (config options, command args, etc.)

see Kernel XIP Instructions For OMAP

Expected Improvement - about .5 seconds
The expected improvement from using this technique depends on the size of the kernel, and the time to load it and decompress it from persistent storage.

In general, time savings of about .5 seconds have been observed.

Projects

 * Configure Linux For XIP describes experience with using both Kernel XIP and application XIP.


 * In this e-mail, David Woodhouse described issues with implementing support for KERNEL XIP in flash. The requirements here are a bit different from supporting KERNEL XIP in ROM, since the flash may be unreadable during certain flash operations.  Therefore, portions of the kernel must be copied to RAM, and certain kernel operations must be disallowed when the flash is unavailable.

Specifications
TODO: list or link to CELF specifications related to this technique

Patches

 * Kernel 2.6.10 now includes XIP support:

ARM PATCH 2154/2: XIP kernel for ARM

Patch from Nicolas Pitre

This patch allows for the kernel to be configured for XIP. A lot of people are using semi hacked up XIP patches already so it is a good idea to have a generic and clean implementation supporting all ARM  targets. The patch isn't too intrusive.

It involves:


 * modifying the kernel entry code to map separate .text and .data sections in the initial page table, as well as relocating .data to ram when needed


 * modifying the linker script to account for the different VMA and LMA for .data, as well as making sure that .init.data gets relocated to ram


 * adding the final kernel mapping with a new MT_ROM mem type


 * distinguishing between XIP and non-XIP for bootmem and memory resource declaration


 * and adding proper target handling to Makefiles.

While at it, this also cleans up the kernel boot code a bit so the kernel can now be compiled for any address in ram, removing the need for a relation between kernel address and start of ram. Also throws in some more comments.

And finally the _text, _etext, _end and similar variables are now declared extern void instead of extern char, or even extern int. That allows for operations on their address directly without any cast, and trying to reference them by mistake would yield an error which is a good thing.

Tested both configurations: XIP and non XIP, the later producing a kernel for execution from ram just as before.

Signed-off-by: Nicolas Pitre Signed-off-by: Russell King

Case 1 - XIP on Arctic III PowerPC board
XIP was used on a PowerPC board, with the following results:


 * Hardware: PowerPC 405LP Arctic III, running at 266 MHZ
 * Kernel Version: MontaVista Linux CEE 3.0 (based on 2.4.20)
 * Configuration: Features built statically into the kernel included: Arctic ethernet, audio, and MTD; 405LP LCD and touchscreen; 405 onchip I2C; and pinned TLBs; Dynamic Power Management; preemptible kernel with selected spinlock breaking; serial driver and serial console (kernel messages are disabled for boot time measurements); TCP/IP (IP addresses are configured after boot) with network devices, network packet filtering, packet protocol, and IP multicast; virtual terminal; UNIX domain sockets and UNIX98 PTYs; Linux Driver Model; and /proc, sysfs, tmpfs, ramfs, cramfs, devpts filesystems.
 * Time without change: 1357 milliseconds
 * Time with change: 894 milliseconds
 * Total Reduction in boot time: 463 milliseconds

Table of bootup times:


 * still have to copy data segment

Thanks to Todd Poynor of MontaVista for providing this information.

Case 2 - XIP on OMAP Innovator
XIP was used on a TI OMAP (Innovator board), with the following results:


 * Hardware: TI OMAP 1510, running at 168 MHZ
 * Kernel Version: 2.4.20 (precursor to CELF tree)
 * Configuration: [need to put config information here]
 * see ["KernelXIPInstructionsForOMAP"]

Thanks to Hiroyuki Machida of Sony for providing this information.

Case 3 - comparing NOR XIP with OneNAND quick-copy to RAM

 * Hardware: TI OMAP 5912, running at 196 MHZ (OSK5912 from Spectrum Digital)
 * Kernel Version: 2.6.10-omap1 (binary size is about 2MBytes uncompressed)

Dongjun Shin of Samsung Electronics reports:

As I've mentioned in AG meeting, we've done some boot time measurements on OMAP 5912 target platform (OSK5912 from Spectrum Digital). We've done this experiment in order to identify the timing gap between NOR XIP and NAND shadowing. Here is the result (the number represents time in microseconds).

The column noted as "XIP tuning" means that we changed the NOR I/F setting of OMAP (EMIFS) so that the synchronous read is used instead of (default) asynchronous read. In case of OneNAND, only 1Kbytes of initial part of OneNAND can be used as XIP region and we used 1Kbytes IPL for loading u-boot. Shadowing means that kernel copy (to RAM) is used.

The reason why the kernel initialization time are broken into 2 phases is that we used timer register for measurement and the timer is initialized during kernel booting. You can just add the values for 2 phases to get the total kernel booting time.


 * Related info
 * omap patch archive
 * Samsung NAND flash memory datasheet

Questions
TimRiker asks:
 * What is the ram/rom footprint of these?
 * Are we close to using sram only for some implementations?
 * Has anyone looked at romfs and XIP user space?

Implementation Notes (from the field)

 * Discussion about XIP when flash might be in use - note mention of '__xipram' attribute (for partial XIP??)

Notes on configuring Linux for XIP (for PPC)

 * Notes on configuring XIP

Using XIP with U-Boot on Arm
Wolfgang Denks, the primary author of the UBoot bootloader, wrote the following:

Lots more details are in the thread (split across months in the archives):
 * Thread: Nov. 2004
 * http://lists.arm.linux.org.uk/pipermail/linux-arm-kernel/2004-December/025674.html Thread: Dec. 2004]

How to determine offsets for sections
I don't know about the ARM in particular, but if you look in ../arch/arm/boot/compressed/vmlinux.lds.in, you will see that this linker-file simply allocates the start addresses of each section as the next available address. The same is true of ../arch/arm/boot/bootp.lds. If you expect to have code the data elements and stack accessed at a specific physical offset, you modify the linker files.

Note that "." means "right here", just like '$' in many assemblers. You can specify a physical offset simply as: ENTRY(_start) SECTIONS {  . == 0x01000000  <==== like this for code .text : { ...   ... }    .rodata : { } . == 0x30000000 <==== like this data .data : { } .bss : {  } } In the above, we have put .rodata (initialized ASCII stuff) right after the code in the .text section. You may need to extract this from the binary blob to put into your NVRAM.

Also, any initialzed data needs to be relocated to your writable SDRAM and the .bss stuff needs to be zeroed. This is non-trivial. You may want to create a ".reloc" section which contains your initialized data, put it in your flash, and relocate it at startup.

Basically executing-in-place is BAD. Flash should exist in some little window where the code gets sucked out, loaded at the correct offset in RAM, then you jump there and close the little window. RAM, even SDRAM,is cheaper than NAND FLASH. You can boot instantly even as I have shown.

Cheers, Dick Johnson