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

[Juergen Gross <jgross@suse.com>]

On 09/15/2014 04:30 PM, David Vrabel wrote:
> On 15/09/14 11:46, Juergen Gross wrote:
>> On 09/15/2014 12:30 PM, David Vrabel wrote:
>>> On 15/09/14 10:52, Juergen Gross wrote:
>>>> On 09/15/2014 11:44 AM, David Vrabel wrote:
>>>>> On 15/09/14 09:52, Juergen Gross wrote:
>>>>>> 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.
>>>>>
>>>>> PV guests can have extra memory added, beyond their initial limit.
>>>>> Supporting this would require option 2.
>>>>
>>>> I don't see why this should require option 2.
>>>
>>> Um...
>>>
>>>> Option 1 only prohibits suspending/migrating a domain with more than
>>>> 512 GB.
>>>
>>> ...this is the reason.
>>>
>>> With the exception of VMs that have assigned direct access to hardware,
>>> migration is an essential feature and must be supported.
>>
>> So you'd prefer:
>>
>> 1) >512GB pv-domains (including Dom0) will be supported only with new
>>     Xen (4.6?), no matter if the user requires migration to be supported
>
> Yes.  >512 GiB and not being able to migrate are not obviously related
> from the point of view of the end user (unlike assigning a PCI device).
>
> Failing at domain save time is most likely too late for the end user.

What would you think about following compromise:

We add a flag that indicates support of multi-level p2m. Additionally
the Linux kernel can ignore the flag not being set either if started as
Dom0 or if told so via kernel parameter.

>
>> to:
>>
>> 2) >512GB pv-domains (especially Dom0 and VMs with direct hw access) can
>>     be started on current Xen versions, migration is possible only if Xen
>>     is new (4.6?)
>
> There's also my preferred option:
>
> 3) >512 GiB PV domains are not supported.  Large guests must be PVH or
> PVHVM.

In theory okay, but not right now, I think. PVH Dom0 is not production
ready.


Juergen

>
>> What is the common use case for migration? I doubt it is used very often
>> for really huge domains.
>
> XenServer uses it for server pool upgrades with no VM downtime.
>
> Also, today's huge VM is tomorrow's common size.
>
> David
>
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