Re: [PATCH v3 2/3] spapr: nvdimm: Implement H_SCM_FLUSH hcall

[Shivaprasad G Bhat]

On 3/25/21 7:21 AM, David Gibson wrote:
> On Wed, Mar 24, 2021 at 09:34:06AM +0530, Aneesh Kumar K.V wrote:
> > On 3/24/21 8:37 AM, David Gibson wrote:
> > > On Tue, Mar 23, 2021 at 09:47:38AM -0400, Shivaprasad G Bhat wrote:
> > > > The patch adds support for the SCM flush hcall for the nvdimm devices.
...
> > > > collects all the hcall states from 'completed' list. The necessary
> > > > nvdimm flush specific vmstate structures are added to the spapr
> > > > machine vmstate.
> > > >
> > > > Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
> > > An overal question: surely the same issue must arise on x86 with
> > > file-backed NVDIMMs.  How do they handle this case?
> > On x86 we have different ways nvdimm can be discovered. ACPI NFIT, e820 map
> > and virtio_pmem. Among these virio_pmem always operated with synchronous dax
> > disabled and both ACPI and e820 doesn't have the ability to differentiate
> > support for synchronous dax.
> Ok.  And for the virtio-pmem case, how are the extra flushes actually
> done on x86?


virtio-pmem device has virtqueue with virtio_pmem_flush() as the handler

which gets called for all flush requests from guest. virtio_pmem_flush() is

offloading the flush to thread pool with a worker doing fsync() and the

completion callback notifying the guest with response.


> > With that I would expect users to use virtio_pmem when using using file
> > backed NVDIMMS
> So... should we prevent advertising an NVDIMM through ACPI or e820 if
> it doesn't have sync-dax enabled?


Is it possible to have different defaults for sync-dax based on 
architecture ?

The behaviour on x86 is sync-dax=on for nvdimms. So, it would be correct to

have the default as "on" for x86. For pseries -  "off" for new machines.

Looking at code, I didnt find much ways to achieve this. Can you suggest

what can be done ?