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

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> Keir Fraser <keir.fraser@xxxxxxxxxxxxx> 18.12.08 12:20 >>>
>I think we should keep the current layout up to the point that the required
>address space wraps past zero. Beyond that we can flag a new layout.
>
>Who's surprised that a domain won't boot with nearly 512GB of memory? It's
>never been tested, and it's not immediately an argument for messing with the
>memory layout, unless it clearly will not fit.

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).

But you didn't respond in any way to this:

>> Are you aware of kernel side dependencies on this layout other than the
>> initial page table setup needing to be made aware of the possibility of
>> the p2m map sitting other than between kernel start and initial page
>> tables end?

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...

Jan


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