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Re: [Xen-devel] dom0 pvops and rearranging memory layout

On 01/23/2015 12:35 PM, Andrew Cooper wrote:
On 23/01/15 10:32, Juergen Gross wrote:

while testing new patches to support dom0 with more than 512 GB I
stumbled over an issue which - I think - is present in pvops for
some time now.

On boot the kernel rearranges the memory layout to match the host
E820 map. This is done to be able to access all I/O areas with
identity mapped pfns (pfn == mfn). So basically some memory pages
change their pfns while the mfns stay the same.

There is no check done whether the moved memory areas are actually
in use (e.g. via memblock_is_reserved()). This can lead to cases
where memory in use is put to an area which is made available for
new memory allocations soon afterwards. Memory in question could
be the initrd, the p2m map presented to dom0 by the hypervisor, or
(hopefully in theory only) even the kernel itself or it's initial
page tables built by the hypervisor.

In my test I had a p2m map of nearly 2GB size and the area between
2GB and 4GB had no RAM. So parts of the p2m map and the complete
initrd where subject to be remapped which led to an early PANIC.

I'll try to add some special handling for the initrd and the p2m
map. In case someone has a better idea: please tell me.

The relocation is done based only on the e820 is it not?


I wonder whether it might be reasonable to extend contruct_dom0/libelf
to avoid constructing a p2m where pfns of built data (kernel, initrd,
p2m and initial pagetables) aliased with host io regions.

That was my first idea, too. OTOH this would require a rather new
hypervisor with this functionality to be able to run a pvops dom0 on
such a machine.

Ans can we be sure that an existing non-pvops dom0 (or even an old pvops
one) can work with such a change?


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