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[Xen-users] 32bit paravirt guests on large memory machines

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Subject: [Xen-users] 32bit paravirt guests on large memory machines
From: Kevin Maguire <kmaguire@xxxxxxx>
Date: Thu, 9 Dec 2010 18:39:02 +0100 (CET)
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Hi Xen-Users

My question relates to this comment in the xend config file on my SLES 11 SP1 installation

--> snip

# 32-bit paravirtual domains can only consume physical
# memory below 168GB. On systems with memory beyond that address,
# they'll be confined to memory below 128GB.
# Using total_available_memory (in GB) to specify the amount of memory reserved
# in the memory pool exclusively for 32-bit paravirtual domains.
# Additionally you should use dom0_mem = <-Value> as a parameter in
# xen kernel to reserve the memory for 32-bit paravirtual domains, default
# is "0" (0GB).
(total_available_memory 0)

--> snip

I have a system with 256 GB of RAM (and many cores). Our intention had been to run a large number of 32 bit paravirtual guests, with each guest having 4 or 8 cores and 16 GB RAM assigned, on that system. The restrictions indicated above pose a problem.

1) Can someone clarify if this restriction is a Xen restriction, which is possible or likely to be lifted/relaxed at some future date? I note that Xen on SLES is based on xen-4.0.0. Or is it a (hardware/addressing/...) limitation that simply cannot be "fixed"?

2) Have I interpreted correctly, in that with Xen 4 I will not be able to use any memory above 128GB for any 32 bit para-virtualised guests, independent of the kernel/config/release of the guest itself? Experiment has led me to suppose my interpretation is correct, but I feel worth double checking?

3) Is the restriction also independent of the Xen hypervisor used, e.g. it does not matter if the hypervisor is 32 bit or 64 bit?


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