Re: [Qemu-devel] Questions about vNVDIMM on qemu/KVM

[Dan Williams]

On Thu, May 24, 2018 at 12:19 AM, Yasunori Goto <address@hidden> wrote:
>> On Tue, May 22, 2018 at 10:08 PM, Yasunori Goto <address@hidden> wrote:
>> > Hello,
>> >
>> > I'm investigating status of vNVDIMM on qemu/KVM,
>> > and I have some questions about it. I'm glad if anyone answer them.
>> >
>> > In my understanding, qemu/KVM has a feature to show NFIT for guest,
>> > and it will be still updated about platform capability with this patch set.
>> > https://lists.gnu.org/archive/html/qemu-devel/2018-05/msg04756.html
>> >
>> > And libvirt also supports this feature with <memory model='nvdimm'>
>> > https://libvirt.org/formatdomain.html#elementsMemory
>> >
>> >
>> > However, virtio-pmem is developing now, and it is better
>> > for archtectures to detect regions of NVDIMM without ACPI (like s390x)
>>
>> I think you are confusing virtio-pmem (patches from Pankaj) and
>> virtio-mem (patches from David)? ...or I'm confused.
>
> Probably, "I" am confusing.
> So, your clarification is very helpful for me.
>
>
>>
>> > In addition, It is also necessary to flush guest contents on vNVDIMM
>> > who has a backend-file.
>>
>> virtio-pmem is a mechanism to use host page cache as pmem in a guest.
>> It does not support high performance memory applications because it
>> requires fsync/msync. I.e. it is not DAX it is the traditional mmap
>> I/O model, but moving page cache management to the host rather than
>> duplicating it in guests.
>
> Ah, ok.
>
>
>>
>> > Q1) Does ACPI.NFIT bus of qemu/kvm remain with virtio-pmem?
>> >     How do each roles become it if both NFIT and virtio-pmem will be 
>> > available?
>> >     If my understanding is correct, both NFIT and virtio-pmem is used to
>> >     detect vNVDIMM regions, but only one seems to be necessary....
>>
>> We need both because they are different. Guest DAX should not be using
>> virtio-pmem.
>
> Hmm. Ok.
>
> But ,I would like understand one more thing.
> In the following mail, it seems that e820 bus will be used for fake DAX.
>
> https://lists.01.org/pipermail/linux-nvdimm/2018-January/013926.html
>
> Could you tell me what is relationship between "fake DAX" in this mail
> and Guest DAX?
> Why e820 is necessary for this case?
>

It was proposed as a starting template for writing a new nvdimm bus
driver. All we need is a way to communicate both the address range and
the flush interface. This could be done with a new SPA Range GUID with
the NFIT, or a custom virtio-pci device that registers a special
nvdimm region with this property. My preference is whichever approach
minimizes the code duplication, because the pmem driver should be
re-used as much as possible.