LinuxVillage welcome => Technical discussions => Discussion démarrée par: melodie le 21 octobre 2013 à 01:58:02

Titre: Fake PAE in Lubuntu
Posté par: melodie le 21 octobre 2013 à 01:58:02


PAE (Physical Address Extension) is explained here in details.

Lubuntu-fake-PAE offers a method to install Lubuntu 13.04 into computers (mainly if not only laptops) with Pentium M and Celeron M CPUs. Most if not all of these CPUs have PAE capability, but show no PAE flag. This means that these machines can not use any Lubuntu versions after 12.04 and therefore, Lubuntu 12.10 and Lubuntu 13.04 can not be installed and their kernels can not be upgraded,

Could the upstart job written by Bernd Kreuss be of any help to you if installed to an Ubuntu distribution?
Here is the content of the script:
description "patch /proc/cpuinfo to add missing pae flag"
author "Bernd Kreuss"

start on runlevel [12345]
stop on runlevel [!12345]

pre-start script
if grep -q " pae " /proc/cpuinfo ; then
echo "This CPU already has the pae flag, doing nothing"
cat /proc/cpuinfo | sed 's/flags\t*:/& pae/' > /tmp/cpuinfo_pae
mount -o bind /tmp/cpuinfo_pae /proc/cpuinfo
mount -o remount,ro,bind /proc/cpuinfo
end script

post-stop script
if grep -q "/tmp/cpuinfo_pae" /etc/mtab ; then
umount /tmp/cpuinfo_pae
rm /tmp/cpuinfo_pae
end script

Of course, the Ubuntu distribution is running to provide distros fit for the newest hardware, and default provide kernels fit for machines having CPUs with PAE.

Now what are these machines which have "pae" but don't have the flag? Where does this come from and how many people have such machines?

I don't have one although we have a pair of old machines at home (the eldest still working came out in 2000 I guess but it's not a Pentium M or a Celeron M in it). Do you have one ?

Titre: Re : Fake PAE in Lubuntu
Posté par: mimas le 21 octobre 2013 à 12:01:57
There is one point that I don't understand. The pseudo file /proc/cpuinfo is provided by linux kernel, and this one halts if the CPU does not match the flags required. So, where is the bug: in linux kernel, in Ubuntu kernel installer/updater?

This script misses a CPU check, it forces PAE to 'exist' on every CPUs not supporting it. This way, it's free PAE for everyone, even for non Intel achitecture based CPU (Arm, PowerPC...).
Titre: Re : Fake PAE in Lubuntu
Posté par: melodie le 22 octobre 2013 à 00:05:39
Hi mimas,

I have ported the discussion and your interesting answer to this forum: Lubuntu et le fake-pae ( Let's see how it goes.  :)