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

[Kevin Maguire <kmaguire@xxxxxxx>]

Hi

No-one any comment/experience/knowledge on the issue below? Should I be 
asking on xen-devel?
Thanks,
Kevin


> 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?
> Thanks,
> Kevin
>
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