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

[Dan Williams]

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.

> 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.

> 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.

>     Otherwize, is the NFIT bus just for keeping compatibility,
>     and virtio-pmem is promising way?
>
>
> Q2) What bus is(will be?) created for virtio-pmem?
>     I could confirm the bus of NFIT is created with <memory model='nvdimm'>,
>     and I heard other bus will be created for virtio-pmem, but I could not
>     find what bus is created concretely.
>     ---
>       # ndctl list -B
>       {
>          "provider":"ACPI.NFIT",
>          "dev":"ndbus0"
>       }
>     ---
>
>     I think it affects what operations user will be able to, and what
>     notification is necessary for vNVDIMM.
>     ACPI defines some operations like namespace controll, and notification
>     for NVDIMM health status or others.
>     (I suppose that other status notification might be necessary for vNVDIMM,
>      but I'm not sure yet...)
>
> If my understanding is wrong, please correct me.

The current plan, per my understanding, is a virtio-pmem SPA UUID
added to the virtual NFIT so that the guest driver can load the pmem
driver but also hook up the virtio command ring for forwarding
WRITE_{FUA,FLUSH} commands as host fsync operations.