Clarification: Is CONFIG_HIGHMEM64G supposed to be set for 64bit Ubuntu Kernels or not?

[Brendan Grainger]

Hi All,

We're running Xen enabled Hardy LTS 8.04 on an 8Gb ASUS P5QL VM  
machine. We are finding that with memory remapping off the OS does not  
see the whole 8Gb, but with memory remapping on it sees 8Gb. I have no  
idea really, but I thought that memory remapping only applied to PAE  
32bit linux. I've seen various posts regarding the CONFIG_HIGHMEM64G  
setting be on in order for Ubuntu to see > 4Gb, but am confused as to  
whether this applies only to PAE enabled 32bit Ubuntu. Is this setting  
required for 64 bit ubuntu also? On our build it is not set:

root@?????:/boot# grep CONFIG_HIGHMEM64G config-2.6.24-2*
root@?????:/boot#

I've put the relevant bits of free -m and lshw below.

Thanks
Brendan


Further details:

# uname -a:
Linux wkr05 2.6.24-25-xen #1 SMP Tue Oct 20 08:47:34 UTC 2009 x86_64  
GNU/Linux

# free -m with memory remapping on:
# free -m
              total       used       free     shared    buffers      
cached
Mem:          7833        350       7483          0          5          
62
-/+ buffers/cache:        281       7551
Swap:        12401          0      12401

# free -m with memory remapping off:

# free -m
              total       used       free     shared    buffers      
cached
Mem:          3104        255       2848          0          4          
60
-/+ buffers/cache:        190       2913
Swap:        12401          0      12401


# Relevant bit of lshw
# lshw

      *-cpu:0
           product: Intel(R) Core(TM)2 CPU          6400  @ 2.13GHz
           vendor: Intel Corp.
           physical id: 1
           bus info: cpu at 0
           width: 64 bits
           capabilities: fpu fpu_exception wp de tsc msr pae mce cx8  
apic sep mtrr mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss  
ht tm pbe syscall nx x86-64 constant_tsc arch_perfmon pebs bts  
rep_good pni monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr lahf_lm
      *-cpu:1
           product: Intel(R) Core(TM)2 CPU          6400  @ 2.13GHz
           vendor: Intel Corp.
           physical id: 2
           bus info: cpu at 1
           width: 64 bits
           capabilities: fpu fpu_exception wp de tsc msr pae mce cx8  
apic sep mtrr mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss  
ht tm pbe syscall nx x86-64 constant_tsc arch_perfmon pebs bts  
rep_good pni monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr lahf_lm


*-memory
           description: System Memory
           physical id: 31
           slot: System board or motherboard
           size: 8GiB
         *-bank:0
              description: DIMM Synchronous 800 MHz (1.2 ns)
              product: ModulePartNumber00
              vendor: Manufacturer00
              physical id: 0
              serial: SerNum00
              slot: DIMM0
              size: 2GiB
              width: 64 bits
              clock: 800MHz (1.2ns)
         *-bank:1
              description: DIMM Synchronous 800 MHz (1.2 ns)
              product: ModulePartNumber01
              vendor: Manufacturer01
              physical id: 1
              serial: SerNum01
              slot: DIMM1
              size: 2GiB
              width: 64 bits
              clock: 800MHz (1.2ns)
         *-bank:2
              description: DIMM Synchronous 800 MHz (1.2 ns)
              product: ModulePartNumber02
              vendor: Manufacturer02
              physical id: 2
              serial: SerNum02
              slot: DIMM2
              size: 2GiB
              width: 64 bits
              clock: 800MHz (1.2ns)
         *-bank:3
              description: DIMM Synchronous 800 MHz (1.2 ns)
              product: ModulePartNumber03
              vendor: Manufacturer03
              physical id: 3
              serial: SerNum03
              slot: DIMM3
              size: 2GiB
              width: 64 bits
              clock: 800MHz (1.2ns)

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