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

[David Gibson]

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.
> > > To be available for exploitation by guest through the next patch.
> > > 
> > > The hcall expects the semantics such that the flush to return
> > > with H_BUSY when the operation is expected to take longer time along
> > > with a continue_token. The hcall to be called again providing the
> > > continue_token to get the status. So, all fresh requsts are put into
> > > a 'pending' list and flush worker is submitted to the thread pool.
> > > The thread pool completion callbacks move the requests to 'completed'
> > > list, which are cleaned up after reporting to guest in subsequent
> > > hcalls to get the status.
> > > 
> > > The semantics makes it necessary to preserve the continue_tokens
> > > and their return status even across migrations. So, the pre_save
> > > handler for the device waits for the flush worker to complete and
> > > 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?

> 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?

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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