Re: [PATCH v4 0/3] nvdimm: Enable sync-dax property for nvdimm

[Dan Williams <dan.j.williams@intel.com>]

On Tue, May 4, 2021 at 2:03 AM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
>
> On 5/4/21 11:13 AM, Pankaj Gupta wrote:
> ....
>
> >>
> >> What this patch series did was to express that property via a device
> >> tree node and guest driver enables a hypercall based flush mechanism to
> >> ensure persistence.
> >
> > Would VIRTIO (entirely asynchronous, no trap at host side) based
> > mechanism is better
> > than hyper-call based? Registering memory can be done any way. We
> > implemented virtio-pmem
> > flush mechanisms with below considerations:
> >
> > - Proper semantic for guest flush requests.
> > - Efficient mechanism for performance pov.
> >
>
> sure, virio-pmem can be used as an alternative.
>
> > I am just asking myself if we have platform agnostic mechanism already
> > there, maybe
> > we can extend it to suit our needs? Maybe I am missing some points here.
> >
>
> What is being attempted in this series is to indicate to the guest OS
> that the backing device/file used for emulated nvdimm device cannot
> guarantee the persistence via cpu cache flush instructions.

Right, the detail I am arguing is that it should be a device
description, not a backend file property. In other words this proposal
is advocating this:

-object memory-backend-file,id=mem1,share,sync-dax=$sync-dax,mem-path=$path
-device nvdimm,memdev=mem1

...and I am advocating for reusing or duplicating the virtio-pmem
model like this:

-object memory-backend-file,id=mem1,share,mem-path=$path
-device spapr-hcall,memdev=mem1

...because the interface to the guest is the device, not the
memory-backend-file.