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[PATCH v3 0/3] spapr: nvdimm: Enable sync-dax property for nvdimm

From: Shivaprasad G Bhat
Subject: [PATCH v3 0/3] spapr: nvdimm: Enable sync-dax property for nvdimm
Date: Tue, 23 Mar 2021 09:47:10 -0400
User-agent: StGit/0.21

The nvdimm devices are expected to ensure write persistence during power
failure kind of scenarios.

The libpmem has architecture specific instructions like dcbf on power
to flush the cache data to backend nvdimm device during normal writes.

Qemu - virtual nvdimm devices are memory mapped. The dcbf in the guest
doesn't traslate to actual flush to the backend file on the host in case
of file backed v-nvdimms. This is addressed by virtio-pmem in case of x86_64
by making explicit flushes translating to fdatasync at qemu.

On PAPR, the issue is addressed by adding a new hcall to
request for an explicit flush from the guest ndctl driver when the backend
nvdimm cannot ensure write persistence with dcbf alone. So, the approach
here is to convey when the hcall flush is required in a device tree
property. The guest makes the hcall when the property is found, instead
of relying on dcbf.

The first patch adds the necessary asynchronous hcall support infrastructure
code at the DRC level. Second patch implements the hcall using the

Hcall number and semantics finalized, so dropping the RFC prefix.

A new device property sync-dax is added to the nvdimm device. When the 
sync-dax is off(default), device property "hcall-flush-required" is set,
and the guest makes hcall H_SCM_FLUSH requesting for an explicit flush. 

By default, sync-dax is "off" on all new pseries machines and prior to
5.2 its "on",

The below demonstration shows the map_sync behavior with sync-dax on & off.

The pmem0 is from nvdimm with With sync-dax=on, and pmem1 is from nvdimm with 
syn-dax=off, mounted as
/dev/pmem0 on /mnt1 type xfs 
/dev/pmem1 on /mnt2 type xfs 

[root@atest-guest ~]# ./mapsync /mnt1/newfile    ----> When sync-dax=off
[root@atest-guest ~]# ./mapsync /mnt2/newfile    ----> when sync-dax=on
Failed to mmap  with Operation not supported

The first patch does the header file cleanup necessary for the
subsequent ones. Second patch implements the hcall, adds the necessary
vmstate properties to spapr machine structure for carrying the hcall
status during save-restore. The nature of the hcall being asynchronus,
the patch uses aio utilities to offload the flush. The third patch adds
the 'sync-dax' device property and enables the device tree property
for the guest to utilise the hcall.

v2 - https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg07031.html
Changes from v2:
      - Using the thread pool based approach as suggested by Greg
      - Moved the async hcall handling code to spapr_nvdimm.c along
        with some simplifications
      - Added vmstate to preserve the hcall status during save-restore
        along with pre_save handler code to complete all ongoning flushes.
      - Added hw_compat magic for sync-dax 'on' on previous machines.
      - Miscellanious minor fixes.

v1 - https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg06330.html
Changes from v1
      - Fixed a missed-out unlock
      - using QLIST_FOREACH instead of QLIST_FOREACH_SAFE while generating token

Shivaprasad G Bhat (3):
      spapr: nvdimm: Forward declare and move the definitions
      spapr: nvdimm: Impletment scm flush hcall
      spapr: nvdimm: Enable sync-dax device property for nvdimm

 hw/core/machine.c             |    1 
 hw/mem/nvdimm.c               |    1 
 hw/ppc/spapr.c                |    6 +
 hw/ppc/spapr_nvdimm.c         |  269 +++++++++++++++++++++++++++++++++++++++++
 include/hw/mem/nvdimm.h       |   10 ++
 include/hw/ppc/spapr.h        |   12 ++
 include/hw/ppc/spapr_nvdimm.h |   34 +++--
 7 files changed, 317 insertions(+), 16 deletions(-)


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