Re: [RFC PATCH] libnvdimm: Update the meaning for persistence_domain values

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

On Wed, Jan 15, 2020 at 9:31 AM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
>
> On 1/15/20 10:57 PM, Aneesh Kumar K.V wrote:
> > On 1/15/20 10:25 PM, Jeff Moyer wrote:
> >> "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:
> >>
> >>> Currently, kernel shows the below values
> >>>     "persistence_domain":"cpu_cache"
> >>>     "persistence_domain":"memory_controller"
> >>>     "persistence_domain":"unknown"
> >>>
> >>> This patch updates the meaning of these values such that
> >>>
> >>> "cpu_cache" indicates no extra instructions is needed to ensure the
> >>> persistence
> >>> of data in the pmem media on power failure.
> >>>
> >>> "memory_controller" indicates platform provided instructions need to
> >>> be issued
> >>> as per documented sequence to make sure data flushed is guaranteed to
> >>> be on pmem
> >>> media in case of system power loss.
> >>>
> >>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> >>> ---
> >>>   arch/powerpc/platforms/pseries/papr_scm.c | 7 ++++++-
> >>>   include/linux/libnvdimm.h                 | 6 +++---
> >>>   2 files changed, 9 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/arch/powerpc/platforms/pseries/papr_scm.c
> >>> b/arch/powerpc/platforms/pseries/papr_scm.c
> >>> index c2ef320ba1bf..26a5ef263758 100644
> >>> --- a/arch/powerpc/platforms/pseries/papr_scm.c
> >>> +++ b/arch/powerpc/platforms/pseries/papr_scm.c
> >>> @@ -360,8 +360,13 @@ static int papr_scm_nvdimm_init(struct
> >>> papr_scm_priv *p)
> >>>       if (p->is_volatile)
> >>>           p->region = nvdimm_volatile_region_create(p->bus, &ndr_desc);
> >>> -    else
> >>> +    else {
> >>> +        /*
> >>> +         * We need to flush things correctly to guarantee persistance
> >>> +         */
> >>> +        set_bit(ND_REGION_PERSIST_MEMCTRL, &ndr_desc.flags);
> >>>           p->region = nvdimm_pmem_region_create(p->bus, &ndr_desc);
> >>> +    }
> >>>       if (!p->region) {
> >>>           dev_err(dev, "Error registering region %pR from %pOF\n",
> >>>                   ndr_desc.res, p->dn);
> >>
> >> Would you also update of_pmem to indicate the persistence domain,
> >> please?
> >>
> >
> > sure.
> >
> >
> >>> diff --git a/include/linux/libnvdimm.h b/include/linux/libnvdimm.h
> >>> index f2a33f2e3ba8..9126737377e1 100644
> >>> --- a/include/linux/libnvdimm.h
> >>> +++ b/include/linux/libnvdimm.h
> >>> @@ -52,9 +52,9 @@ enum {
> >>>        */
> >>>       ND_REGION_PERSIST_CACHE = 1,
> >>>       /*
> >>> -     * Platform provides mechanisms to automatically flush outstanding
> >>> -     * write data from memory controler to pmem on system power loss.
> >>> -     * (ADR)
> >>> +     * Platform provides instructions to flush data such that on
> >>> completion
> >>> +     * of the instructions, data flushed is guaranteed to be on pmem
> >>> even
> >>> +     * in case of a system power loss.
> >>
> >> I find the prior description easier to understand.
> >
> >
> > I was trying to avoid the term 'automatically, 'memory controler' and
> > ADR. Can I update the above as
> >
> > /*
> >   * Platform provides mechanisms to flush outstanding write data
> >   * to pmem on system power loss.
> >   */
> >
>
> Wanted to add more details. So with the above interpretation, if the
> persistence_domain is found to be 'cpu_cache', application can expect a
> store instruction to guarantee persistence.

The expectation is globally visible stores are persisted

> If it is 'none' there is no
> persistence ( I am not sure how that is the difference from 'volatile'
> pmem region).

"None" means the "platform does not enumerate a persistence domain".
That's not necessarily "volatile" because the user may know that they
have battery backup, or some other private/out-of-band capability not
exported by the platform. In which case they would manually need to
manipulate the pmemX/write_cache property manually.

> If it is  'memory_controller' ( I am not sure whether that
> is the right term), application needs to follow the recommended
> mechanism to flush write data to pmem.

If it is memory_controller the expectation is that flushing data out
of the cpu caches and making those writebacks to memory globally
visible is sufficient for the platform to persist flushed data on a
power loss.
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