[SOLVED] UEFI - LEGACY boot clarification

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overkill22
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[SOLVED] UEFI - LEGACY boot clarification

Postby overkill22 » Thu Nov 09, 2017 11:41 pm

Hi guys,
I'd like to have a clarification because I can't understand something.

I have a new laptop >2016 that is an ASUS ZENBOOK. It came with Windows 8 or 10, that was immediately wiped out in favour of Mint Cinnamon.
During the installation, it says something about UEFI and BIOS and stuff like that.

At the moment my bios is with:
SECURE BOOT >> disabled,
FAST BOOT >> enabled,
LAUNCH CSM >> disabled.
(here a picture not mine of what it looks like, my bios is updated 2015 but looks the same https://i.stack.imgur.com/kepiA.jpg)

Now my question.
For some reasons* I had to disable all those options, fast boot and launch csm too. What happens or what's the difference between having fast boot disabled and launch csm enabled?


*the reasons I had to do that is because I'll need to boot from USB a distro Puppy Linux that it seems to work only in "legacy mode". After many trials I found that "legacy mode" is enabled by disabling all those above.
Last edited by overkill22 on Mon Nov 13, 2017 12:46 am, edited 1 time in total.

fabien85
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Re: UEFI - LEGACY boot clarification

Postby fabien85 » Fri Nov 10, 2017 4:54 am

Hi,
fast boot is a feature where the firmware skips a few steps during the Power-On Self Test (POST, the thing that happens at the very beginning when you press the power button, before you get the splash screen of the manufacturer) to provide faster boot time. It's something completely unrelated to UEFI or Legacy, and poses absolutely no problem in general.
The thing is there is often a confusion with a windows feature called fast startup, which is problematic for dual-boot windows/linux (or windows/macOS or windows/whatever).

CSM stands for Compatibility Support Module and is the thing that allows to boot a Legacy/BIOS bootloader on a UEFI computer. So it needs to be enabled in order to boot something in Legacy mode.

Secure boot is a UEFI feature whereby the computer checks the signature of the boot loader against some signature (called secure boot keys) stored in the firmware, before launching the boot loader. It's something that can be problematic for linux, depending on which keys the manufacturer has provided, but that's a whole other story.
When this feature is activated, the boot loader must be signed (with a good signature) ; because Legacy boot loaders cannot be signed (they are not a well-defined file in a filesystem, just data blocks in the Master Boot Record of the drive), some manufacturer interpret that activating secure boot should disable Legacy boot loaders, since they cannot be checked. So for some computers, secure boot must be disabled in order to boot something in Legacy mode.

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overkill22
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Re: UEFI - LEGACY boot clarification

Postby overkill22 » Sun Nov 12, 2017 10:48 pm

fabien85 wrote:Hi,
fast boot is a feature where the firmware skips a few steps during the Power-On Self Test (POST, the thing that happens at the very beginning when you press the power button, before you get the splash screen of the manufacturer) to provide faster boot time. It's something completely unrelated to UEFI or Legacy, and poses absolutely no problem in general.
The thing is there is often a confusion with a windows feature called fast startup, which is problematic for dual-boot windows/linux (or windows/macOS or windows/whatever).

CSM stands for Compatibility Support Module and is the thing that allows to boot a Legacy/BIOS bootloader on a UEFI computer. So it needs to be enabled in order to boot something in Legacy mode.

Secure boot is a UEFI feature whereby the computer checks the signature of the boot loader against some signature (called secure boot keys) stored in the firmware, before launching the boot loader. It's something that can be problematic for linux, depending on which keys the manufacturer has provided, but that's a whole other story.
When this feature is activated, the boot loader must be signed (with a good signature) ; because Legacy boot loaders cannot be signed (they are not a well-defined file in a filesystem, just data blocks in the Master Boot Record of the drive), some manufacturer interpret that activating secure boot should disable Legacy boot loaders, since they cannot be checked. So for some computers, secure boot must be disabled in order to boot something in Legacy mode.


Thank you very much, this explanation was so clear and good that I totally understand what is what!

In this case, it is ok and what is the difference if I keep the CSM on? Would it be better, same, or worst? If worst, why?

mr_raider
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Re: UEFI - LEGACY boot clarification

Postby mr_raider » Mon Nov 13, 2017 12:38 am

overkill22 wrote:

In this case, it is ok and what is the difference if I keep the CSM on? Would it be better, same, or worst? If worst, why?


There is no "danger" in keeping legacy boot. Legacy boot allows you to boot devices in the older legacy method.

If you're Mint was installed in UEFI mode, legacy boot is only needed to boot stuff like your puppy Linux.

No matter what you do with setting, always know if your are booted in legacy or UEFI mode. Misidentifying the boot mode can lead to installation problems.
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