Re: [Xen-devel] [RFC KERNEL PATCH 0/2] Add Dom0 NVDIMM support for Xen

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

On 20/10/2016 10:14, Haozhong Zhang wrote:
>
>>>>>
>>>>>> Once dom0 has a mapping of the nvdimm, the nvdimm driver can go to
>>>>>> work
>>>>>> and figure out what is on the DIMM, and which areas are safe to use.
>>>>> I don't understand this ordering of events.  Dom0 needs to have a
>>>>> mapping to even write the on-media structure to indicate a
>>>>> reservation.  So, initial dom0 access can't depend on metadata
>>>>> reservation already being present.
>>>>
>>>> I agree.
>>>>
>>>> Overall, I think the following is needed.
>>>>
>>>> * Xen starts up.
>>>> ** Xen might find some NVDIMM SPA/MFN ranges in the NFIT table, and
>>>> needs to note this information somehow.
>>>> ** Xen might find some Type 7 E820 regions, and needs to note this
>>>> information somehow.
>>>
>>> IIUC, this is to collect MFNs and no need to create frame table and
>>> M2P at this stage. If so, what is different from ...
>>>
>>>> * Xen starts dom0.
>>>> * Once OSPM is running, a Xen component in Linux needs to collect and
>>>> report all NVDIMM SPA/MFN regions it knowns about.
>>>> ** This covers the AML-only case, and the hotplug case.
>>>
>>> ... the MFNs reported here, especially that the former is a subset
>>> (hotplug ones not included in the former) of latter.
>>
>> Hopefully nothing.  However, Xen shouldn't exclusively rely on the dom0
>> when it is capable of working things out itself, (which can aid with
>> debugging one half of this arrangement).  Also, the MFNS found by Xen
>> alone can be present in the default memory map for dom0.
>>
>
> Sure, I'll add code to parsing NFIT in Xen to discover statically
> plugged pmem mode NVDIMM and their MFNs.
>
> By the default memory map for dom0, do you mean making
> XENMEM_memory_map returns above MFNs in Dom0 E820?

Potentially, yes.  Particularly if type 7 is reserved for NVDIMM, it
would be good to report this information properly.

>
>>>
>>> (There is no E820 hole or SRAT entries to tell which address range is
>>> reserved for hotplugged NVDIMM)
>>>
>>>> * Dom0 requests a mapping of the NVDIMMs via the usual mechanism.
>>>
>>> Two questions:
>>> 1. Why is this request necessary? Even without such requests like what
>>>   my current implementation, Dom0 can still access NVDIMM.
>>
>> Can it?  (if so, great, but I don't think this holds in the general
>> case.)  Is that a side effect of the NVDIMM being covered by a hole in
>> the E820?
>
> In my development environment, NVDIMM MFNs are not covered by any E820
> entry and appear after RAM MFNs.
>
> Can you explain more about this point? Why can it work if covered by
> E820 hole?

It is a question, not a statement.  If things currently work fine then
great.  However,  there does seem to be a lot of flexibility in how the
regions are reported, so please be mindful to this when developing the code.

>
>>
>>>
>>> 2. Who initiates the requests? If it's the libnvdimm driver, that
>>>   means we still need to introduce Xen specific code to the driver.
>>>
>>>   Or the requests are issued by OSPM (or the Xen component you
>>>   mentioned above) when they probe new dimms?
>>>
>>>   For the latter, Dan, do you think it's acceptable in NFIT code to
>>>   call the Xen component to request the access permission of the pmem
>>>   regions, e.g. in apic_nfit_insert_resource(). Of course, it's only
>>>   used for Dom0 case.
>>
>> The libnvdimm driver should continue to use ioremap() or whatever it
>> currently does.  There shouldn't be Xen modifications like that.
>>
>> The one issue will come if libnvdimm tries to ioremap()/other an area
>> which Xen is unaware is an NVDIMM, and rejects the mapping request.
>> Somehow, a Xen component will need to find the MFN/SPA layout and
>> register this information with Xen, before the ioremap() call made by
>> the libnvdimm driver.  Perhaps a notifier mechanism out from the ACPI
>> subsystem might be the best way to make this work in a clean way.
>>
>
> Yes, this is necessary for hotplugged NVDIMM.

Ok.

>
>>>
>>>> ** This should work, as Xen is aware that there is something there
>>>> to be
>>>> mapped (rather than just empty physical address space).
>>>> * Dom0 finds that some NVDIMM ranges are now available for use
>>>> (probably
>>>> modelled as hotplug events).
>>>> * /dev/pmem $STUFF starts happening as normal.
>>>>
>>>> At some pointer later after dom0 policy decisions are made
>>>> (ultimately,
>>>> by the host administrator):
>>>> * If an area of NVDIMM is chosen for Xen to use, Dom0 needs to inform
>>>> Xen of the SPA/MFN regions which are safe to use.
>>>> * Xen then incorporates these regions into its idea of RAM, and starts
>>>> using them for whatever.
>>>>
>>>
>>> Agree. I think we may not need to fix the way/format/... to make the
>>> reservation, and instead let the users (host administrators), who have
>>> better understanding of their data, make the proper decision.
>>
>> Yes.  This is the best course of action.
>>
>>>
>>> In a worse case that no reservation is made, Xen hypervisor could turn
>>> to use RAM for management structures for NVDIMM, with the cost of less
>>> RAM for guests.
>>
>> Or simply not manage the NVDIMM at all.
>>
>> OTOH, a different usecase might be to register a small area for Xen to
>> use to crash log into.
>>
>
> an interesting usage, but I'd like to put it in the future work.

Absolutely.  I didn't wish to suggest implementing this now.  It was
just pointing out an alternative usecase.

Leaving this for future work will be perfectly fine.

~Andrew
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