how linux deals with bios settings
 

i have some questions about what bios settings linux does and does not disregard at boot time.

it seems that some are sacrosanct, and others are not.

for example, most of the settings for the southbridge and lower seem to be disregarded. linux re enumerates the hardware and initializes all the settings separately, almost like a plug and play operation.

but, something like Hyperthreading seems sacrosanct: if it is off in the bios, linux doesn't automatically enable it.

are there any hard and fast rules about what settings in the bios linux respects vs disregards? am i completely missing an important theme?

thanks for any help

it is a good question and forgive me for my answer.

If you are having trouble with something, try amending your kernel append line in your grub or lilo file or
try re-compiling your kernel with or without module support for that certain something.

2) Altho I am a quasi expert, Grub the bootloader accepts LBA if enabled in the bios but I still use kernel appends myself.
I have plug n play turned off on my old intel 440bx mobo but I also don't have fancy USB devices that I need to start on boot.

3) maybe you could be more specific with what you want enabled or what errors you get?

Hyperthreading is a form of symmetrical multiprocessing. This requires an SMP kernel, which will utilize it if available. Check if your kernel is built with SMP support (they are usually called this-or-that-version-smp).


Håkan

thanks very much for both replies.

my question started out a little more general than your answers. i'm not experiencing any problems with a linux boot (in my case, a knoppix boot). in fact, the problem is the other way around. i was experiencing a boot problem with a display device that was evident while viewing the bios boot screen, while booted into memtest86 or dos, and while trying to boot into win2k. BUT, ALL the errors went away when i booted a knoppix live cd. since win2k wouldn't boot at all, it was a lifesaver, and i've been trying to figure out why it worked.

i think i've come to a conclusion about that, and if you're interested, i can fill you in. it is a really good success story with linux..

anyway, what came from this troubleshooting process is more questions concerning how linux interacts with the bios (generally) and what bios settings are overridden or ignored in a regular linux boot. and now, finally, it comes to: what bios settings can be explicitly overridden.

i found this little message: (garbled so that i can post it, just remove the spaces and you should reach the page)
www . ussg.iu.edu/hypermail/linux/kernel/0011.3/0275. html

which seems to indicate (although somewhat flippantly) that anything is possible. so, according to the poster, regardless of what settings you are using for the bios init, you can override them, possibly by booting a customized copy of linux.

but, i wonder if that's really true. specifically, is it true with hyperthreading?

and i guess these are my questions now:

1) is it possible to override the hyperthreading option (enable/disable) set in the bios?

2) has anyone actually done this?


i think it would be very interesting to see a linux boot that was able to automatically or manually (through a text interface) set these options at boot. like: if hyperthreading is available, force it to be off or on, or force all peripheral port devices (serial, parallel, usb, ieee) to be enabled or disabled at boot. or even enable/disable disk controllers.

do you know if this is possible, and if anyone has actually done it?

1) http://www.linuxquestions.org/questi...hreadid=237511
shows grub the boot loader and menu.lst examples of the kernel command

eg kernel /boot/vmlinuz root=/dev/hda3 ro
the ro is the kernel append means read only until fully booted

other appends include noprobe noscsi etc

EDIT oops it appears you need a smp kernel so edit of kernel line needs a whole new kernel named and its initrd image

so you need to amend grub to have 2 entries
one for a normal kernel and one for smp kernel

However, the above kernel appends will still allow other hardware changes.

2) Slightly different and messier is the amendment on a booted system of your startup scripts in /etc/rc.d/
Mine are bsd style so simple to read don't know about yours.

3) you may care to check google
http://www.google.com.au/linux?hl=en...e+Search&meta=

and one result of search LQ gave this
http://www.linuxquestions.org/questi...reading+kernel

redo search with your distro etc

Adding what hw-tph said. The UMP (single processor) kernel does not turn off hyperthreading. It just see the first processor not the second. You will see a reduce cache because of this. Windows XP uses Windows 2000 core, so it is able handle two processors. Win98 can not handle any more than one processor. In your kernel parameters documentation will have some options you can include in your bootloader, so it can pass them to the kernel.

The Linux kernel bypasses the BIOS and goes directly to low-level to get information by itself. This way Linux can be stable at low-level and at high-level (programs). For an examle, if the BIOS sees a 120 GB hard drive as 50 GB, Linux is able to read the full 120 GB of the drive ignoring what the BIOS tells it. Linux does not have brains to boot to drives, so it has to depend on the BIOS and the installed bootloader.

If you have a problem starting up Windows, you will have to troubleshoot it in safe-mode or re-install it. I do know for a fact that Linux can easily be fix without re-installing. Windows users are brain washed to re-install Windows when the system does not boot or something is corrupted. In Linux, problems will occur but re-installing Linux hurts your learning experience. Files (configs) can easliy be reconstructed if they are corrupted.

thanks for the answers guys.

do either of you know if the -noht boot switch actually forces hyperthreading off, just as if it was switched off in the bios (no performance hit relative to explicit bios off)? i can't find a straight answer either way

It should turn off hyperthreading. It does not hurt it try it out.

thanks one last time, i'll try various things out