HVM and free_memory query

[Apoorva Kulkarni <apoorva@3tera.com>]

Hello All

I have tried the xen-users list for this query but haven't received any input there yet so trying here. I have an Intel Xeon server with 32G memory. My setup is xen-3.2.2 with 32-bit CentOS 5.1 dom0 with a 64-bit hypervisor. 

Initial state...
#xm info
<snip>
release                : 2.6.18.8-xen0
version                : #1 Thu Jun 18 14:06:08 PDT 2009
machine                : i686
nr_cpus                : 8
nr_nodes               : 1
cores_per_socket       : 4
threads_per_core       : 1
cpu_mhz                : 1861
hw_caps                : bfebfbff:20100800:00000000:00000140:0004e3bd:00000000:00000001
total_memory           : 32766
free_memory            : 31298
node_to_cpu            : node0:0-7
xen_major              : 3
xen_minor              : 2
xen_extra              : .2-rc3-pre
xen_caps               : xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32 hvm-3.0-x86_32p hvm-3.0-x86_64
xen_scheduler          : credit
xen_pagesize           : 4096
platform_params        : virt_start=0xfdc00000
</snip>


#xm list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0   512     1     r-----  25216.9
controller                                   1   512     1     r-----  39912.5


After I create a Windows 2003 Server VM with 30G RAM...

#xm info
<snip>
total_memory           : 32766
free_memory            : 337
</snip>

#xm list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0   512     1     r-----  25251.4
controller                                   1   512     1     -b----  39948.6
vm.windows03                                44 30720     1     r-----     22.0

As you can see, the free_memory shown is 337M which is a good 241M less than what it should have been (31298M - 30720M = 578M) after the Windows VM was created. My question here is where/why does xen use that extra 241M. Any pointers much appreciated.

TIA!
-- Apoorva Kulkarni 3Tera, Inc.