Re: [Xen-devel] 32-bit PV guests and 168 GB boundary

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.04.13 at 16:21, Nick Pegg <npegg@xxxxxxxxxx> wrote:
> 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?

No, this is an interface limitation: 32-bit PV guests can't be expected
to be able to deal with an address space hole of more than 168Mb
at the top of the address space, yet a bigger hole would be needed
to make the full (read-only) M2P table for the guest.

If you wanted to lift that limit, you'd have to make the guest capable
of falling back to using hypercalls for doing the M2P lookups for MFNs
outside the visible range - no one cared enough about this to
implement something like that.

Jan


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