linux-kernel - Re: [Xen-devel] incorrect layout of globals from head

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Date:	Tue, 10 Jul 2012 14:32:13 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Olaf Hering <olaf@...fle.de>
Cc:	Daniel Kiper <dkiper@...-space.pl>, xen-devel@...ts.xensource.com,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Jan Beulich <JBeulich@...e.com>
Subject: Re: [Xen-devel] incorrect layout of globals from head_64.S during
 kexec boot

On Tue, Jul 10, 2012 at 08:09:53PM +0200, Olaf Hering wrote:
> 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

Ewww.
> 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.

Sure. And we could scan it.. but scanning E820_UNKNOWN for some magic
header seems .. hacky.

> 
> > > 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.

What about an populate_physmap right afterwards to stick a newly
minted GMFN in its place? I don't really know whether this dance
of balloon out/balloon in the same GMFN will break the shared_info
relationship. Perhaps not?

What we are going for is to stop the hypervisor from using the shared_info
MFN... perhaps there are other ways to do this?
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