Re: [Xen-devel] [PATCH V3 1/1] expand x86 arch_shared_info to support >3 level p2m tree

[Juergen Gross <jgross@xxxxxxxx>]

On 09/15/2014 10:29 AM, Andrew Cooper wrote:
> On 12/09/2014 11:31, Juergen Gross wrote:
> > On 09/09/2014 12:49 PM, Juergen Gross wrote:
> > > On 09/09/2014 12:27 PM, Andrew Cooper wrote:
> > > > On 09/09/14 10:58, Juergen Gross wrote:
> > > > > The x86 struct arch_shared_info field pfn_to_mfn_frame_list_list
> > > > > currently contains the mfn of the top level page frame of the 3 level
> > > > > p2m tree, which is used by the Xen tools during saving and restoring
> > > > > (and live migration) of pv domains. With three levels of the p2m tree
> > > > > it is possible to support up to 512 GB of RAM for a 64 bit pv domain.
> > > > > A 32 bit pv domain can support more, as each memory page can hold 1024
> > > > > instead of 512 entries, leading to a limit of 4 TB. To be able to
> > > > > support more RAM on x86-64 an additional level is to be added.
> > > > >
> > > > > This patch expands struct arch_shared_info with a new p2m tree root
> > > > > and the number of levels of the p2m tree. The new information is
> > > > > indicated by the domain to be valid by storing ~0UL into
> > > > > pfn_to_mfn_frame_list_list (this should be done only if more than
> > > > > three levels are needed, of course).
> > > > A small domain feeling a little tight on space could easily opt for a 2
> > > > or even 1 level p2m.  (After all, one advantage of virt is to cram many
> > > > small VMs into a server).
> > > >
> > > > How is xen and toolstack support for n-level p2ms going to be
> > > > advertised
> > > > to guests?  Simply assuming the toolstack is capable of dealing with
> > > > this new scheme wont work with a new pv guest running on an older Xen.
> > > Is it really worth doing such an optimization? This would save only very
> > > few pages.
> > >
> > > If you think it should be done we can add another SIF_* flag to
> > > start_info->flags. In this case a domain using this feature could not be
> > > migrated to a server with old tools, however. So we would probably end
> > > with the need to be able to suppress that flag on a per-domain base.
> > Any further comments?
> >
> > Which way should I go?
> There are two approaches, with different up/downsides
>
> 1) continue to use the old method, and use the new method only when
> absolutely required.  This will function, but on old toolstacks, suffer
> migration/suspend failures when the toolstack fails to find the p2m.
>
> 2) Provide a Xen feature flag indicating the presence of N-level p2m
> support.  Guests which can see this flag are free to use N-level, and
> guests which can't are not.
>
> Ultimately, giving more than 512GB to a current 64bit PV domain is not
> going to work, and the choice above depends on which failure mode you
> wish a new/old mix to have.
I'd prefer solution 1), as it will enable Dom0 with more than 512 GB
without requiring a change of any Xen component. Additionally large
domains can be started by users who don't care for migrating or
suspending them.

So I'd rather keep my patch as posted.


Juergen


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