Re: 32-bit PV guests and 168 GB boundary

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 15/04/13 15:21, Nick Pegg wrote:
> Hello,
>
> I've recently ran into a problem on a physical machine with a large
> amount of memory where 32-bit PV guests would refuse to boot with this
> error message:
>
> xm create: Error: (4, 'Out of memory', "panic: xc_dom_boot.c:159:
> xc_dom_boot_mem_init: can't allocate low memory for domain")
>
> We discovered that there's a boundary where 32-bit PV guests can't be
> allocated memory above 168GB, along with the work-around config option
> to reserve memory for these guests (total_available_memory in
> xend-config.sxp).
>
> Could someone clarify why this is a restriction? Is this some sort of
> hardware limitation?
>
>
> Thanks,
> Nick

This is because of the amount of the Xen m2p table mapped into 32bit
kernel space, between 0xf5800000 and 0xffffffff.  PV guests require the
m2p table to perform paravirtualised memory management.  This is a
fundamental limit of 32bit PV guests.

~Andrew

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