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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Jan 23, 2015 at 11:32:20AM +0100, 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

Oh my. That is huge. Could you compress it? This would require of course
a new type of P2M - where would mark which MFNs are contingous.

And then during booting you could read over and find these special
ones and when creating the new P2M do the right uncompression?

> 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.
> 
> 
> Juergen

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