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

[Pankaj Gupta]

> > The proposal that "sync-dax=unsafe" for non-PPC architectures, is a
> > fundamental misrepresentation of how this is supposed to work. Rather
> > than make "sync-dax" a first class citizen of the device-description
> > interface I'm proposing that you make this a separate device-type.
> > This also solves the problem that "sync-dax" with an implicit
> > architecture backend assumption is not precise, but a new "non-nvdimm"
> > device type would make it explicit what the host is advertising to the
> > guest.
> >
>
> Currently, users can use a virtualized nvdimm support in Qemu to share
> host page cache to the guest via the below command
>
> -object memory-backend-file,id=memnvdimm1,mem-path=file_name_in_host_fs
> -device nvdimm,memdev=memnvdimm1
>
> Such usage can results in wrong application behavior because there is no
> hint to the application/guest OS that a cpu cache flush is not
> sufficient to ensure persistence.
>
> I understand that virio-pmem is suggested as an alternative for that.
> But why not fix virtualized nvdimm if platforms can express the details.
>
> ie, can ACPI indicate to the guest OS that the device need a flush
> mechanism to ensure persistence in the above case?
>
> 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.

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.

> >> On PPC, the default is "sync-dax=writeback" - so the ND_REGION_ASYNC
> >>
> >> is set for the region and the guest makes hcalls to issue fsync on the 
> >> host.
> >>
> >>
> >> Are you suggesting me to keep it "unsafe" as default for all architectures
> >>
> >> including PPC and a user can set it to "writeback" if desired.
> >
> > No, I am suggesting that "sync-dax" is insufficient to convey this
> > property. This behavior warrants its own device type, not an ambiguous
> > property of the memory-backend-file with implicit architecture
> > assumptions attached.
> >
>
> Why is it insufficient?  Is it because other architectures don't have an
> ability express this detail to guest OS? Isn't that an arch limitations?