Difference between revisions of "Fallback mechanism"

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== Introduction ==
+
== Introduction ==
TODO
+
This mechanism permits to test and recover from certain non-booting coreboot images.
  
== Howto ==
+
This works by having two coreboot images in the same flash chip:
* build the coreboot image as usual, it will produce an image in build/coreboot.rom
+
* One fallback/ image: The working image.
* After the first build run:
+
* One normal/ image: The image to be tested.
  make menuconfig
+
 
* Go in  
+
This feature is not widely tested on all boards. It also requires it to have a reboot_counter exported in the CMOS layout.
General setup  --->
+
 
* Change:
+
This also doesn't protect against human errors when using such feature, or bugs in the code responsible for switching between the two images.
 +
 
 +
== Uses cases ==
 +
* Test new images way faster: if the image doesn't boot it will fallback on the old known-working image and save a long reflashing procedure. Handy for bisecting faster.
 +
* Test new images more safely: Despite of the recommendations of having a way to externally reflash, many new user don't. Still, this method is not totally foolproof.
 +
* More compact testing setup: Since reflashing tools are not mandatory anymore, the tests can be done with less hardware, very useful when traveling.
 +
 
 +
== How it works ==
 +
Coreboot increments a reboot count at each boot but never clears it. What runs after coreboot is responsible for that.
 +
 
 +
That way, the count can be cleared by the OS once it's fully booted.
 +
 
 +
If a certain threshold<ref>Defined by CONFIG_MAX_REBOOT_CNT, typically 3</ref> is attained at boot, coreboot will boot the fallback image.
 +
 
 +
== Warnings ==
 +
Because we uses two images, it's easy to wrongly identify which image booted:
 +
* If the user mistakenly thinks the normal image is booting...
 +
* But the fallback image always boots...
 +
* And the normal image doesn't work...
 +
* And the user flashes the normal in fallback because she thinks it boots fine...
 +
* Then the user bricked her device and has to reflash it externally.
 +
 
 +
== Fallback build ==
 +
To configure it for fallback, do:
 +
  $ make menuconfig
 +
Then in "General setup  --->", near the top use "fallback" in "CBFS prefix to use":
 
  (fallback) CBFS prefix to use
 
  (fallback) CBFS prefix to use
To:
+
Then near the bottom, make sure to have:
 +
[ ] Update existing coreboot.rom image
 +
And in the "Chipset  --->" menu at the bottom:
 +
Bootblock behaviour (Switch to normal if CMOS says so)  --->
 +
[*] Do not clear reboot count after successful boot
 +
 
 +
You can then build the fallback image with the [[Fallback mechanism/fallback.sh|fallback.sh]] script.
 +
 
 +
== Normal build ==
 +
To configure it for normal, do:
 +
$ make menuconfig
 +
Then in "General setup  --->", near the top use "normal" in "CBFS prefix to use":
 
  (normal) CBFS prefix to use
 
  (normal) CBFS prefix to use
* Go back to the main menu and select
+
Then near the bottom, make sure to have:
Architecture (x86)  --->
 
select the following option:
 
 
  [*] Update existing coreboot.rom image
 
  [*] Update existing coreboot.rom image
Exit and save and rebuild...
+
And in the "Chipset  --->" menu at the bottom:
 +
Bootblock behaviour (Switch to normal if CMOS says so)  --->
 +
[*] Do not clear reboot count after successful boot
 +
 
 +
You can then build with the normal part with the [[Fallback mechanism/normal.sh|normal.sh]] script. It takes an existing coreboot image as argument.
 +
 
 +
== OS configuration ==
 +
 
 +
=== The manual way ===
 +
An approach is to run switch-to-normal.sh before trying an image.
 +
It's however more error prone than the systemd approach because:
 +
* you have to do it manually, each time, before testing an image.
 +
* If you then want to use that new image, you have to flash it, again, to fallback.
 +
 
 +
==== switch-to-normal.sh ====
 +
#!/bin/sh
 +
nvramtool -w boot_option=Normal
 +
nvramtool -w reboot_counter=0
 +
 
 +
==== switch-to-fallback.sh ====
 +
#!/bin/sh
 +
nvramtool -w boot_option=Fallback
 +
nvramtool -w reboot_counter=15
 +
 
 +
(Assuming that 15 is the maximum that can be stored in reboot_counter.)
 +
 
 +
=== Systemd ===
 +
Here we use systemd to automatically reset the boot counter after each successful boot (or resume).
 +
 
 +
We are then supposed to use the normal image daily and only resort to fallback in case of issues.
 +
 
 +
To install it, first install nvramtool (from coreboot sources):
 +
$ cd util/nvramtool
 +
$ make
 +
$ sudo make install
 +
 
 +
Then add the following systemd units at their respective paths:
 +
* [[Fallback_mechanism/coreboot@boot.service|/etc/systemd/system/coreboot@boot.service]]
 +
* [[Fallback_mechanism/coreboot@resume.service|/etc/systemd/system/coreboot@resume.service]]
 +
 
 +
Then enable them with:
 +
$ sudo systemctl enable coreboot@boot.service
 +
$ sudo systemctl start coreboot@boot.service
 +
$ sudo systemctl enable coreboot@resume.service
 +
$ sudo systemctl start coreboot@resume.service
 +
 
 +
== Current limitations ==
 +
* '''Use of the same cmos.layout in fallback and normal !'''
 +
* The user may wrongly identify which image booted, and because of that, end up reflashing a non-working image.
 +
* Some issues can arrise when the nvram layout is not the same between normal/ and fallback/
 +
* The number of failed boot is fixed at compilation time.
 +
* In order to fully boot, some boards do reset conditionally during the boot process resulting in a non-predictable increment of the boot count.
 +
* Example script exist only for systemd. Still, they are trivial to adapt to other init systems.
 +
* Payloads sometime have fixed default locations when loading things from cbfs:
 +
** When using grub as a payload, grub.cfg is at etc/grub.cfg by default, so if you want to test grub as a payload, remember to change grub.cfg's path not to interfer with the fallback's grub configuration.
 +
** Changing the path of what SeaBIOS loads from cbfs is probably configurable with SeaBIOS cbfs symlinks but not yet tested/documented with the use of the fallback mecanism
 +
* Tested boards need to be listed somewhere.
 +
 
 +
== Issues ==
 +
=== thinkpad_acpi ===
 +
This linux driver can have some bad interactions with the fallback/normal mecanism: when using it with the volume_control=1 option, volume_mode=1 is required, otherwise after shutting down the computer, it will always boot from fallback.
 +
 
 +
This might be because as the default settings of volume_mode touches the nvram, it probably corrupts it at shutdown when saving the alsa state of the volume buttons "sound card" (called EC Mixer). Then at boot, coreboot will detects a corrupted nvram and restore its valid defaults.
  
The image will then have fallback and normal:
+
== references ==
Name                          Offset    Type        Size
+
<references/>
cmos_layout.bin                0x0        cmos_layout  1776
 
pci1002,9710.rom              0x740      optionrom    60928
 
fallback/romstage              0xf580    stage        92823
 
fallback/coreboot_ram          0x26080    stage        66639
 
fallback/payload              0x36540    payload      54976
 
config                        0x43c40    raw          4455
 
normal/romstage                0x44e00    stage        92823
 
normal/coreboot_ram            0x5b8c0    stage        68820
 
normal/payload                0x6c600    payload      159949
 
(empty)                        0x93700    null        442136
 

Latest revision as of 20:59, 25 February 2018

Introduction

This mechanism permits to test and recover from certain non-booting coreboot images.

This works by having two coreboot images in the same flash chip:

  • One fallback/ image: The working image.
  • One normal/ image: The image to be tested.

This feature is not widely tested on all boards. It also requires it to have a reboot_counter exported in the CMOS layout.

This also doesn't protect against human errors when using such feature, or bugs in the code responsible for switching between the two images.

Uses cases

  • Test new images way faster: if the image doesn't boot it will fallback on the old known-working image and save a long reflashing procedure. Handy for bisecting faster.
  • Test new images more safely: Despite of the recommendations of having a way to externally reflash, many new user don't. Still, this method is not totally foolproof.
  • More compact testing setup: Since reflashing tools are not mandatory anymore, the tests can be done with less hardware, very useful when traveling.

How it works

Coreboot increments a reboot count at each boot but never clears it. What runs after coreboot is responsible for that.

That way, the count can be cleared by the OS once it's fully booted.

If a certain threshold[1] is attained at boot, coreboot will boot the fallback image.

Warnings

Because we uses two images, it's easy to wrongly identify which image booted:

  • If the user mistakenly thinks the normal image is booting...
  • But the fallback image always boots...
  • And the normal image doesn't work...
  • And the user flashes the normal in fallback because she thinks it boots fine...
  • Then the user bricked her device and has to reflash it externally.

Fallback build

To configure it for fallback, do:

$ make menuconfig

Then in "General setup --->", near the top use "fallback" in "CBFS prefix to use":

(fallback) CBFS prefix to use

Then near the bottom, make sure to have:

[ ] Update existing coreboot.rom image

And in the "Chipset --->" menu at the bottom:

Bootblock behaviour (Switch to normal if CMOS says so)  --->
[*] Do not clear reboot count after successful boot

You can then build the fallback image with the fallback.sh script.

Normal build

To configure it for normal, do:

$ make menuconfig

Then in "General setup --->", near the top use "normal" in "CBFS prefix to use":

(normal) CBFS prefix to use

Then near the bottom, make sure to have:

[*] Update existing coreboot.rom image

And in the "Chipset --->" menu at the bottom:

Bootblock behaviour (Switch to normal if CMOS says so)  --->
[*] Do not clear reboot count after successful boot

You can then build with the normal part with the normal.sh script. It takes an existing coreboot image as argument.

OS configuration

The manual way

An approach is to run switch-to-normal.sh before trying an image. It's however more error prone than the systemd approach because:

  • you have to do it manually, each time, before testing an image.
  • If you then want to use that new image, you have to flash it, again, to fallback.

switch-to-normal.sh

#!/bin/sh
nvramtool -w boot_option=Normal
nvramtool -w reboot_counter=0

switch-to-fallback.sh

#!/bin/sh
nvramtool -w boot_option=Fallback
nvramtool -w reboot_counter=15

(Assuming that 15 is the maximum that can be stored in reboot_counter.)

Systemd

Here we use systemd to automatically reset the boot counter after each successful boot (or resume).

We are then supposed to use the normal image daily and only resort to fallback in case of issues.

To install it, first install nvramtool (from coreboot sources):

$ cd util/nvramtool
$ make
$ sudo make install

Then add the following systemd units at their respective paths:

Then enable them with:

$ sudo systemctl enable coreboot@boot.service
$ sudo systemctl start coreboot@boot.service
$ sudo systemctl enable coreboot@resume.service
$ sudo systemctl start coreboot@resume.service

Current limitations

  • Use of the same cmos.layout in fallback and normal !
  • The user may wrongly identify which image booted, and because of that, end up reflashing a non-working image.
  • Some issues can arrise when the nvram layout is not the same between normal/ and fallback/
  • The number of failed boot is fixed at compilation time.
  • In order to fully boot, some boards do reset conditionally during the boot process resulting in a non-predictable increment of the boot count.
  • Example script exist only for systemd. Still, they are trivial to adapt to other init systems.
  • Payloads sometime have fixed default locations when loading things from cbfs:
    • When using grub as a payload, grub.cfg is at etc/grub.cfg by default, so if you want to test grub as a payload, remember to change grub.cfg's path not to interfer with the fallback's grub configuration.
    • Changing the path of what SeaBIOS loads from cbfs is probably configurable with SeaBIOS cbfs symlinks but not yet tested/documented with the use of the fallback mecanism
  • Tested boards need to be listed somewhere.

Issues

thinkpad_acpi

This linux driver can have some bad interactions with the fallback/normal mecanism: when using it with the volume_control=1 option, volume_mode=1 is required, otherwise after shutting down the computer, it will always boot from fallback.

This might be because as the default settings of volume_mode touches the nvram, it probably corrupts it at shutdown when saving the alsa state of the volume buttons "sound card" (called EC Mixer). Then at boot, coreboot will detects a corrupted nvram and restore its valid defaults.

references

  1. Defined by CONFIG_MAX_REBOOT_CNT, typically 3