Re: [Xen-users] request for explanation of acpi=off

[Ernst Bachmann <e.bachmann@xxxxxxxx>]

I'm not exactly a specialist on PC hardware, but here my try at an 
explanation:

ACPI is basically a way for the hardware to tell the OS how it works, the BIOS 
provides tables with information about what resources are available, how 
they're wired together, etc.

Unfortunately, lots of mainboard vendors don't care much about the "quality" 
of their tables, even if they're plainly wrong, they count as "good enough" 
if the windows HAL parses them into something usable.

acpi=off tells xen and/or the kernel to ignore those tables.
Instead, it tries to "guess" what is there by using PNP bios,  PCI-pnp, and 
assuming the board is PC-XT compatible.

if those guesses are correct for the functions you need to use, you won't need 
acpi.

Now, as a side effect, the APIC (Advanced programmable interrupt controller) 
won't be found without ACPI, so unless there's a way to force APIC use 
without ACPI, you'll loose the APIC features, meaning you'll have to live 
with the old PC-XT interrupt features (two pics with 8 IRQ-Lines each, 
cascaded on IRQ2)

Now if your hardware has special interrupt needs and you don't mind several 
devices sharing the same IRQ, you won't need APIC.

Still, my advice would be to try to get acpi working, upgrading to a current 
BIOS version often helps.

/Ernst

On Friday 20 January 2006 11:17, Henning Sprang wrote:

...
> I get those problems now also at home, where I can't spend money on
> hardware until I find working hardware by accident, and the hardware
> posted as known to work is also over my budget - so I like to use that
> machine with acpi=off, but I want to know what the consequences can be.
...
> > - why exactly am I not supposed to use acpi=off?
> >
> > - what can/will happen when I do so?
> >
> > - does the discouragement of acpi=off absolutely "forbid" me to use it
> > in stable production environments, or is it just "not optimal" to do it,
> > but acceptable if there's no other (easy) way to get some hardware
> > running with xen and there's not enough money to buy as much hardware
> > until I have some which does the acpi stuff well with xen?

_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users