[Nexus 7 Developer Discussion [Developers Only]] Tasssadar:
In this post, I would like to explain what kexec-hardboot patch is and also bring it to light a bit more since until now, it was only burried in MultiROM thread.

@kernel developers: I would like to ask you to merge this patch to your kernels, because it is essential part of MultiROM - it allows me to boot any kernel without changing the boot partition. I realize that it is no small request, but the patch is not big, touches relatively stable parts of kernel and should not cause any problems. Thank you.

What is kexec?

It is syscall of Linux kernel, which allows you to boot another Linux kernel without restarting the device - "Linux boots itself". The functionality is equivalent to fastboot -c *cmdline* boot zImage initrd.img, but without PC and fastboot. It is fairly known thing, so more info at wikipedia and man kexec.

Standard kexec call unfortunatelly does not work on Nexus 7. It freezes somewhere, and it is very difficult to find out where - probably some of the drivers are not shut down/re-initialized properly, it is a commong thing among Android devices, which is why kexec-hardboot was made.

What is the difference between normal and hardboot exec?

Kexec-hardboot patch adds a real device restart to that process, so that all the drivers can be properly reinitialized. It stores new kernel to RAM, reboots the device as usual, and kernel from boot partition immediately jumps to the one which was stored to RAM before reboot.

Another difference is that both kernels must be patched. The "host" kernel requires a full patch, the one which is being kexecd' requires only two small compatibility patches.

To sumarize the process:

kexec --load-hardboot.... is called and kernel it loaded into RAM.

kexec -e is called. Special info is written to memory (to area which is not overwritten on reboot) and the device is rebooted.

After reboot, very early in the boot process, kernel checks if that special info is present in RAM and if so, it loads new kernel from RAM and jumps to it.

Kexecd' kernel starts and boots.

For more info, read the original thread.

Patches:

Full kernel patch: https://gist.github.com/4558647, 4.1 kernel repo, cm 10.1 kernel repo

This is the full kernel patch. Kernels with this patch can be both "host" and "guest" kernels.

Related CONFIG options:

CONFIG_KEXEC=y

CONFIG_KEXEC_HARDBOOT=y

CONFIG_ATAGS_PROC=y

CONFIG_GROUPER_HARDBOOT_RECOVERY=n

First three options must be enabled. Last one, GROUPER_HARDBOOT_RECOVERY, specifies if the kexec call should reboot to normal mode or to recovery. This can be useful if you don't have the kernel in boot partition but only as kernel in recovery partition. You usualy want to disable this option.
​

Compatibility patch: https://gist.github.com/4458581

This patch only makes the kernel bootable via kexec, ie. it can't be the host kernel. This was made for Ubuntu kernel (and it was accepted), because I did not want to drag the whole patch in there. If your kernel is for Android ROMs, you should use the full patch.​

Userspace kexec binary: View attachment 1653562

That ZIP file contains kexec source, patches and README. It is from the original patch from Mike Kasick. It also contains precompiled, statically linked kexec binary, so you probably just want to use that.​

Usage:

Once you have the kernel patches and kexec userspace binary in place, just run following command to boot into new kernel:

kexec --load-hardboot zImage --initrd=initrd.img --mem-min=0x85000000 --command-line="$(cat /proc/cmdline)"
kexec -e

Note the command line parameter - cmdline from bootloader is not added automatically, you have to put it there by yourself.

Currently used by:

MultiROM - I use it to boot different kernels (eg. Ubuntu).

MOSLO - Part of Plasma Active for Nexus 7, also usefull tool for every N7 developer - read more: http://ruedigergad.com/2012/12/09/nexus-7-easily-flashaccess-entire-userdata-partition/

Authors:

This patch was made by Mike Kasick for Samsung Epic 4G. Since that, it was ported to several devices, one of them is Asus Transformer TF201 - I used patch from TF201 and modified it a bit (basically just changed few SoC specific constants). People at #ubuntu-arm helped me out with that, thanks.