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Re: [Libreboot] message during boot: "DMAR: Failed to map dmar2"

From: Daniel Tarrero
Subject: Re: [Libreboot] message during boot: "DMAR: Failed to map dmar2"
Date: Fri, 13 Nov 2015 11:56:09 +0100


i also had seen DMAR warnings too when dealing with virtualization (in
hosts and guests systems).

Since BIOS support virtualization, dynamic mapping address at the OS
level doesnt seem to be a great idea... Think that, in that case we have
multiple OS's, each one with its (virtual) bios and (virtual) address
mapping, but all talking to the same hardware!

Sometimes you want virtual OS to talk through virtual devices that talk
to real hardware, other times you want virtual OS to talk straight to
the hardware.

So the party seem different here, you see.

A different approach trying to solve this. Not sure, but i think it's in
the "virtualization platform" hands to map this. In the KVM platform i
use, there's a module which handles this and other stuff too.

My explanations are far from exact and techy, but i hope it helps you
with your computer :) and correct me if im so far from reality!! 8)

Regards and nice weekend!!!!!!

El jue, 12-11-2015 a las 08:25 -0500, Bruno Dantas escribió:
> It seems that interrupt mapping is done by BIOS (or Libreboot in my case) 
> first, then the linux kernel remaps things for whatever reason [0]. I know 
> nothing about this low-level stuff, but because the laptop is working despite 
> the DMAR failure, I ventured a guess that the interrupt remapping done by the 
> kernel is not strictly necessary.
> Therefore, I looked at the complete list of kernel boot parameters [1] and 
> tried all the ones that sounded promising. I found that intremap=off makes 
> the boot-time DMAR error go away! I tested all peripherals and everything 
> seems to be working fine. Relevant lines from dmesg here: 
> In some cases, this DMAR error is related to faulty chipsets [2]. I guess the 
> question that remains is whether my T400 (or its Libreboot) is defective in 
> some way or is the DMAR failure something that is expected to happen on every 
> T400 with Libreboot? If there is nothing wrong with my hardware or my 
> Libreboot flash, is booting with intremap=off ill-advised for any reason?
> [0] 
> [1]
> [2]

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