Comment 9 for bug 189269

Some further info:

I have an ASUS P5B-E Plus. I've just installed/upgraded to Hardy 64bit server version, also running Xen.

I was experiencing the same proiblem: memory remapping enabled and BIOS reporting 4Gb. But when Hardy was running it was only reporting 2.5Gb.

The fix had nothing to do with the various kernel flags discussed. It's a problem with grub. Follow the instructions described here:

http://www.howtoforge.com/make-your-xen-pae-kernel-work-with-more-than-4gb-ram-debian-etch-grub

(Even though it's for debian etch instead of hardy, the instructions are identical).

I've just followed these instructions and rebooted and now all my memory is visible.

The key to the fix is to modify around line 143 of stage2/common.c thus (just one cavaet-I've created this diff after getting it working and I think the build process may have added lines, I think the link above has the correct line numbers - follow it's directions not mine ;-) ):

--- common.c 2008-06-19 22:52:14.000000000 +1000
+++ common.c.orig 2008-06-19 22:51:42.000000000 +1000
@@ -159,7 +159,8 @@
 init_bios_info (void)
 {
 #ifndef STAGE1_5
- unsigned long cont, memtmp, addr;
+ unsigned long memtmp, addr;
+ volatile unsigned long cont;
   int drive;
 #endif

After making this change, rebuild grub, the debian packages, install the debian package, thne do a grub-install.

Hope this helps others.