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 followed by explicit flushes if the backend devices are not synchronous DAX capable. Qemu - virtual nvdimm devices are memory mapped. The dcbf in the guest and the subsequent flush 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 fsync at qemu. On SPAPR, 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. A new device property sync-dax is added to the nvdimm device. When the sync-dax is 'writeback'(default for PPC), device property "hcall-flush-required" is set, and the guest makes hcall H_SCM_FLUSH requesting for an explicit flush. sync-dax is "unsafe" on all other platforms(x86, ARM) and old pseries machines prior to 5.2 on PPC. sync-dax="writeback" on ARM and x86_64 is prevented now as the flush semantics are unimplemented. When the backend file is actually synchronous DAX capable and no explicit flushes are required, the sync-dax mode 'direct' is to be used. The below demonstration shows the map_sync behavior with sync-dax writeback & direct. (https://github.com/avocado-framework-tests/avocado-misc-tests/blob/master/memory/ndctl.py.data/map_sync.c) The pmem0 is from nvdimm with With sync-dax=direct, and pmem1 is from nvdimm with syn-dax=writeback, mounted as /dev/pmem0 on /mnt1 type xfs (rw,relatime,attr2,dax=always,inode64,logbufs=8,logbsize=32k,noquota) /dev/pmem1 on /mnt2 type xfs (rw,relatime,attr2,dax=always,inode64,logbufs=8,logbsize=32k,noquota) [root@atest-guest ~]# ./mapsync /mnt1/newfile ----> When sync-dax=unsafe/direct [root@atest-guest ~]# ./mapsync /mnt2/newfile ----> when sync-dax=writeback 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. The kernel changes to exploit this hcall is at https://github.com/linuxppc/linux/commit/75b7c05ebf9026.patch --- v3 - https://lists.gnu.org/archive/html/qemu-devel/2021-03/msg07916.html Changes from v3: - Fixed the forward declaration coding guideline violations in 1st patch. - Removed the code waiting for the flushes to complete during migration, instead restart the flush worker on destination qemu in post load. - Got rid of the randomization of the flush tokens, using simple counter. - Got rid of the redundant flush state lock, relying on the BQL now. - Handling the memory-backend-ram usage - Changed the sync-dax symantics from on/off to 'unsafe','writeback' and 'direct'. Added prevention code using 'writeback' on arm and x86_64. - Fixed all the miscellaneous comments. v2 - https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg07031.html Changes from v2: - Using the thread pool based approach as suggested - 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: Implement H_SCM_FLUSH hcall nvdimm: Enable sync-dax device property for nvdimm hw/arm/virt.c | 28 ++++ hw/i386/pc.c | 28 ++++ hw/mem/nvdimm.c | 52 +++++++ hw/ppc/spapr.c | 16 ++ hw/ppc/spapr_nvdimm.c | 285 +++++++++++++++++++++++++++++++++++++++++ include/hw/mem/nvdimm.h | 11 ++ include/hw/ppc/spapr.h | 11 +- include/hw/ppc/spapr_nvdimm.h | 27 ++-- qapi/common.json | 20 +++ 9 files changed, 455 insertions(+), 23 deletions(-) -- Signature
