[Xen-devel] Re: issue with domains having close to or more than 512G

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 18/12/2008 12:51, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> Making wrapping the condition for a change doesn't mean Linux can go
> unfixed, whereas if we set the condition lower (e.g. needing more than one
> L3 table entry, which is where things currently break) the fixes in Linux can
> be restricted to just make the new layout work. In any case, we got a bug
> report on 1Tb not working with Dom0, so we need to do something about
> it (beyond intermediately telling them to restrict Dom0 to about 500G,
> and perhaps making older Xen simply not give out more than what Linux
> currently can cope with).

It will mean that non-Linux PV guests may need 'fixing' for a new layout
when they otherwise wouldn't. Hence we should stretch the old layout as far
as it'll go, at least for non-Linux PV.

> I was actually hoping to get some agreement on the route to go here, so
> I could try to implement a solution acceptable to all. One of the issues I
> can't deal with just by myself is that I'm not really clear what requirements
> non-Linux Dom0 have. For Linux alone, I'd put the p2m map into the
> address space covered by the second from last L4 table entry...

If you simply want the p2m mapped put elsewhere, I think the guest should
advertise that location in its Elf headers. Then in fact the builder can
always use the new layout if it sees the new Elf header format. That allows
Linux to use the new layout in all cases (including the buggy case where
more than one L3 is used) while not needlessly risking breakage for
non-Linux guests.

 -- Keir



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