Re: [Xen-devel] [PATCH 3/3] xen: eliminate scalability issues from initial mapping setup

[Juergen Gross <jgross@xxxxxxxx>]

On 09/04/2014 04:43 PM, Andrew Cooper wrote:
> On 04/09/14 15:31, Jan Beulich wrote:
> > > > > On 04.09.14 at 15:02, <andrew.cooper3@xxxxxxxxxx> wrote:
> > > On 04/09/14 13:59, David Vrabel wrote:
> > > > On 04/09/14 13:38, Juergen Gross wrote:
> > > > > Direct Xen to place the initial P->M table outside of the initial
> > > > > mapping, as otherwise the 1G (implementation) / 2G (theoretical)
> > > > > restriction on the size of the initial mapping limits the amount
> > > > > of memory a domain can be handed initially.
> > > > The three level p2m limits memory to 512 GiB on x86-64 but this patch
> > > > doesn't seem to address this limit and thus seems a bit useless to me.
> > > Any increase of the p2m beyond 3 levels will need to come with
> > > substantial libxc changes first.  3 level p2ms are hard coded throughout
> > > all the PV build and migrate code.
> > No, there no such dependency - the kernel could use 4 levels at
> > any time (sacrificing being able to get migrated), making sure it
> > only exposes the 3 levels hanging off the fourth level (or not
> > exposing this information at all) to external entities making this
> > wrong assumption.
> >
> > Jan
> That would require that the PV kernel must start with a 3 level p2m and
> fudge things afterwards.
I always thought the 3 level p2m is constructed by the kernel, not by
the tools.

It starts with the linear p2m list anchored at xen_start_info->mfn_list,
constructs the p2m tree and writes the p2m_top_mfn mfn to
HYPERVISOR_shared_info->arch.pfn_to_mfn_frame_list_list

See comment in the kernel source arch/x86/xen/p2m.c

So booting with a larger p2m list can be handled completely by the
kernel itself.

> At a minimum, I would expect a patch to libxc to detect a 4 level PV
> guest and fail with a meaningful error, rather than an obscure "m2p
> doesn't match p2m for mfn/pfn X".
I'd rather fix it in a clean way.

I think the best way to do it would be an indicator in the p2m array
anchor, e.g. setting 1<<61 in pfn_to_mfn_frame_list_list. This will
result in an early error with old tools:
"Couldn't map p2m_frame_list_list"


Juergen

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