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[Xen-devel] Xen 3.0 x86 + PAE with > 16 GB RAM?

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Subject: [Xen-devel] Xen 3.0 x86 + PAE with > 16 GB RAM?
From: Derrik Pates <demon@xxxxxxxxxxxxx>
Date: Fri, 06 Jan 2006 17:21:10 -0500
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Xen developers:

My employers are interested in deploying large (specifically Sun Fire
V40z) systems for use as virtual private server hosting platforms using
Xen. However, in playing with Xen on another Opteron based system (with
only 5 GB of RAM), I noticed that the PAE-enabled Xen build claims a 16
GB RAM ceiling. Some poking around in the source of the hyperkernel
(specifically xen/include/asm/config.h) revealed that MACHPHYS_MBYTES
seems to be the key limiting value, capping the system RAM at 16 gigabytes.

Can this safely be increased? From my reading of the code, it appears
that it is set as it is because each gigabyte of installed RAM requires
a megabyte of RAM to be set aside for mapping tables and such. If I just
increase the value of MACHPHYS_MBYTES to 32 (for example), should Xen
boot and work happily, just consuming more RAM for its mapping tables?

Please Cc: me with your reply, as I'm not subscribed to this list currently.

Derrik Pates

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