Re: [Xen-devel] incorrect layout of globals from head_64.S during kexec boot

[Olaf Hering <olaf@aepfle.de>]

On Tue, Jul 10, Konrad Rzeszutek Wilk wrote:

> On Tue, Jul 10, 2012 at 05:23:08PM +0200, Olaf Hering wrote:
> > I was not thinking of statically allocated pages but some new concept of
> > allocating such shared pages. Shouldnt there be some dedicated area in
> > the E820 table which has to be used during the whole life time of the
> > guest?
> 
> Not that I can see. But I don't see why that could not be added? Perhaps
> the HVM loader can make it happen? But then how would it tell the kernel
> that this E820_RESERVED is the shared_info one. Not the other ones..

Maybe just use a new E820 type for this sort of thing? Its just the
question wether some other OS can cope with an unknown type. From my
reading of the e820 related code a region with an unknown type is just
ignored.

> > Are there more shared areas or is it just the shared info page?
> > 
> > > And I am kind of worried that moving it to the .data section won't
> > > be completly safe - as the decompressor might blow away that part too.
> > 
> > The decompressor may just clear the area, but since there is no way to
> > tell where the shared pages are its always a risk to allocate them at
> > compile time.
> 
> Yeah, and with the hypervisor potentially still updating the "old"
> MFN before the new kernel has registered the new MFN, we can end up
> corrupting the new kernel. Ouch.
> 
> Would all of these issues disappear if the hypervisor had a hypercall
> that would stop updating the shared info? or just deregister the MFN?
> What if you ripped the GMFN out using 'decrease_reservation' hypercall?
> Would that eliminate the pesky GMFN?

I'm not sure, most likely the gfn will just disappear from the guest,
like a ballooned page disappears. Accessing it will likely cause a
crash.

Olaf