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

On 23/01/15 10:32, Juergen Gross wrote:
> Hi,
> 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.

I would suggest:

Pass 1: relocate p2m and page tables to "safe" areas.
Pass 2: relocate frames from holes/reserved regions.

I don't think we want to change the hypervisor to workaround this.


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