Re: [Xen-devel] address space reorganization

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 13 Apr 2005, at 18:59, Gerd Knorr wrote:

> If we touch the address space anyway we might fix some other
> issues along the way.  Ian mentioned he wants to move the
> ioremap area to the bottom.  I guess next to the ro MPT
> table, so it's easy to grant domains read-only access to
> ACPI tables?
Ian was talking about the ioremap area in XenLinux, not in Xen. Doing 
this for XenLinux would be useful if we were to make HYPERVISOR_START a 
run-time, rather than compile-time, constant. However, I am not 
convinced that this is a very good idea (see my reasoning below).
That given, it is also not clear that reorg'ing the Xen address space 
is a worthwhile effort. Yes, things like the location of the Xen heap 
will be different on PAE vs. non-PAE builds, but it will still be a 
constant decided at compile time.  Reorg'ing is hopefully a pain we can 
do without for the time being. We should revisit these considerations 
if/when we want a single Xen binary that can do both PAE and non-PAE. 
But let's just get the basic support working first. :-)
> Is it possible (and/or useful) to make the address layout
> dynamic?  So the size of the frametable can be adjusted at
> boot time depending on the amount of memory installed in the
> machine?  That would imply the ro MPT doesn't have a fixed
> address any more, not sure this is possible ...
I do not think that we should have run-time selected 
HYPERVISOR_VIRT_START. This is because it will make it impossible to 
migrate a guest to a machine with more memory than the one on which it 
is currently executing.
The reasoning is: the new target machine will have a lower 
HYPERVISOR_VIRT_START, but the guest will have sized its lowmem area 
according to the available space on the original machine. It therefore 
cannot run  on the new target because its lowmem area simply will not 
fit.
I think we should just fix on two VIRT_START values: -64MB for non-PAE 
and something like -192MB for PAE (or whatever allows us to map up to 
16GB -- I think we will treat bigger memory configs than that as rare 
enough to ignore).
 -- Keir


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