linux-kernel - Re: [Xen-devel] [PATCH v4] xen PVonHVM: move shared

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Date:	Tue, 30 Oct 2012 17:30:06 +0100
From:	Olaf Hering <olaf@...fle.de>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	xen-devel <xen-devel@...ts.xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH v4] xen PVonHVM: move shared_info to reserved
 memory area

On Tue, Oct 30, Jan Beulich wrote:

> >>> On 30.10.12 at 16:47, Olaf Hering <olaf@...fle.de> wrote:
> > This is a respin of 00e37bdb0113a98408de42db85be002f21dbffd3
> > ("xen PVonHVM: move shared_info to MMIO before kexec").
> > 
> > Currently kexec in a PVonHVM guest fails with a triple fault because the
> > new kernel overwrites the shared info page. The exact failure depends on
> > the size of the kernel image. This patch moves the pfn from RAM into an
> > E820 reserved memory area.
> 
> One thing that occurred to me only now: How is this relocation
> of the shared info going to help with the vCPU info placement?
> You can't undo this, nor can you re-register these areas to be
> put in a different location (of course, both of there could be
> implemented in the hypervisor). Yet the hypervisor writes to
> some of these areas' fields as much as it does write to the
> shared info structure itself.

Maybe the wording "move" is a bit misleading in this patch. 

A single "move" of the actual pfn happens during boot, that is when a
PVonHVM enabled guest kernel does the XENMAPSPACE_shared_info operation.
It moves the pfn of the shared info page from the location the hvmloader
initially configured to this new pfn (0xfffff -> 0xfe700).
Another relocation does not happen at runtime, AFAIK.

The "move" which this patch does is more a source level move in the
sense that RESERVE_BRK (which is somewhere in the middle of RAM) is not
used anymore. Instead a pfn in an E820_Reserved area is used, see
xen-unstable changeset 26108:79185dcdf558 "hvmloader: Reserve
FE700000-FE800000 in physical memory map for guest use."

Olaf
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