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

[Juergen Gross <jgross@xxxxxxxx>]

On 01/23/2015 12:35 PM, Andrew Cooper wrote:
> 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.
> The relocation is done based only on the e820 is it not?
Yes.

> 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?


Juergen

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