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xen-devel
Re: [Xen-devel] address space reorganization
On 13 Apr 2005, at 20:29, Keir Fraser wrote:
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).
I chatted to Christian a bit about this and he changed my mind. There
probably are some situations where a variable virt_start would be
useful for us, although we still may not want to do it for an initial
pae patch.
We need generally to think about how flexible we want to be in allowing
migration between different machine configurations. Shoudl we require
identical h/w specs, or allow differences in I/O devices, CPU and/or
memory? We will already have to be careful about downgrading cpu specs
when we migrate (e.g., Linux locks onto using multimedia instructions
for software raid that are unavailable post-migrate). A pragmatic
middleground may be that, if people want to migrate in a heterogeneous
cluster, we require them to configure 'worst-case specs' up front when
building a domain (e.g., lowest-spec cpu the domain should run on;
biggest hypervisor address-space hole the domain should work with).
-- Keir
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