Messages in this thread | | | Date | Fri, 05 Sep 2014 09:55:47 +0200 | From | Juergen Gross <> | Subject | Re: [Xen-devel] [PATCH 3/3] xen: eliminate scalability issues from initial mapping setup |
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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@citrix.com> 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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