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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2013-04-15 at 15:21 +0100, 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?

It's a 32-bit PV guest limitation which arises due to the need to map
the m2p table into each PV guest, there is simply only enough room for
168GB worth of m2p in the 32-bit kernel address space before it starts
to impinge on other stuff. In theory the host admin could be given the
flexibility to make it a little bigger (not sure if any toolstack
implements this), but not by much.

There are various hacky things which could in theory be done, such as
giving 32-bit guests views onto different 168GB windows of physical
memory from which all their RAM must come, but none of those have been
implemented AFAIK.

Hopefully PVH will eventually support 32-bit and this restriction will
go away.

Ian.


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