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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 05/09/14 08:55, Juergen Gross wrote:
> 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.

Ah yes - I remember now.  All the toolstack does is create the linear
p2m.  In which case building such a domain will be fine.

>
>>
>> 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"

No it wont.  The is_mapped() macro in the toolstack is quite broken.  It
stems from a lack of Design/API/ABI concerning things like the p2m.  In
particular, INVALID_MFN is not an ABI constant, nor is any notion of
mapped vs unmapped.

Its current implementation is a relic of 32bit days, and only checks bit
31.  It also means that it is impossible to migrate a PV VM with pfns
above the 43bit limit; a restriction which is lifted by my migration v2
series.  A lot of the other migration constructs are in a similar state,
which is why they are being deleted by the v2 series.

The clean way to fix this is to leave pfn_to_mfn_frame_list_list as
INVALID_MFN. Introduce two new fields beside it named p2m_levels and
p2m_root, which then caters for levels greater than 4 in a compatible
manner.

~Andrew