Weird RAM handling with Xen 3.4.3-RC3

[Thomas Goirand <thomas@goirand.fr>]

Hi,

Using the latest Debian SID Xen kernel (pv_ops 2.6.32-4) that I compiled
in Lenny, I had weird RAM handling issues. Let me explain.

If in grub, I put:

kernel          /boot/xen-3.4-amd64.gz dom0_mem=2048M

then when starting KDM, it gets stuck and doesn't work. If I press e in
grub, remove the dom0_mem part, then my laptop is able to boot and start
X window. Quite annoying, because I quite want to use the directive. It
might be linked to the experimental version of the graphic driver that
I'm using, but I also read that others had issues with X, so I thought
this could be interesting.

Then another very strange behavior. Once I start using xend, I do:

root@GPLHost:buzzig>_ ~# xm info
total_memory           : 3990
free_memory            : 131
root@GPLHost:buzzig>_ ~# xm list
Name     ID   Mem VCPUs  State    Time(s)
Domain-0  0  3800     2  r-----   54.8
root@GPLHost:buzzig>_ ~# xm mem-set Domain-0 2048
root@GPLHost:buzzig>_ ~# xm list
Name     ID   Mem VCPUs  State    Time(s)
Domain-0  0  2048     2  r-----   56.3
root@GPLHost:buzzig>_ ~# xm info
total_memory           : 3990
free_memory            : 1113

After I set my dom0 to use 2048 MB of RAM, I get only 1113 MB of free
RAM. It as if a part of the RAM (829 MB of RAM) has gone! Quite wacko
isn't it? Could it be that the memory management is broken somehow? What
is going on here?

I posted the above to the Debian pkg-xen list, and Ian suggested me to
also send it here.

Thomas