If the device backend is not persistent memory for the nvdimm, there is need for explicit IO flushes to ensure persistence. On SPAPR, the issue is addressed by adding a new hcall to request for an explicit flush from the guest when the backend is not pmem. So, the approach here is to convey when the hcall flush is required in a device tree property. The guest once it knows the device needs explicit flushes, makes the hcall as and when required. It was suggested to create a new device type to address the explicit flush for such backends on PPC instead of extending the generic nvdimm device with new property. So, the patch introduces the spapr-nvdimm device. The new device inherits the nvdimm device with the new bahviour such that if the backend has pmem=no, the device tree property is set by default. The below demonstration shows the map_sync behavior for non-pmem backends. (https://github.com/avocado-framework-tests/avocado-misc-tests/blob/master/memory/ndctl.py.data/map_sync.c) The pmem0 is from spapr-nvdimm with with backend pmem=on, and pmem1 is from spapr-nvdimm with pmem=off, 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 pmem=on [root@atest-guest ~]# ./mapsync /mnt2/newfile ----> when pmem=off Failed to mmap with Operation not supported First patch adds the realize/unrealize call backs to the generic device for the new device's vmstate registration. The 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 introduces the spapr-nvdimm device, adds the device tree property for the guest when spapr-nvdimm is used with pmem=no on the backend. Also adds new property pmem-override(?, suggest if you have better name) to the spapr-nvdimm which hints at forcing the hcall based flushes even on pmem backed devices. The kernel changes to exploit this hcall is at https://github.com/linuxppc/linux/commit/75b7c05ebf9026.patch --- v6 - https://lists.gnu.org/archive/html/qemu-devel/2022-02/msg00322.html Changes from v6: - Addressed commen from Daniel. Fixed a typo Fetch the memory backend FD in the flush_worker_cb(), updated hcall return values in the comments description) - Updated the signatures. v5 - https://lists.gnu.org/archive/html/qemu-devel/2021-07/msg01741.html Changes from v5: - Taken care of all comments from David - Moved the flush lists from spapr machine into the spapr-nvdimm device state structures. So, all corresponding data structures adjusted accordingly as required. - New property pmem-overrride is added to the spapr-nvdimm device. The hcall flushes are allowed when pmem-override is set for the device. - The flush for pmem backend devices are made to use pmem_persist(). - The vmstate structures are also made part of device state instead of global spapr. - Passing the flush token to destination during migration, I think its better than finding, deriving it from the outstanding ones. v4 - https://lists.gnu.org/archive/html/qemu-devel/2021-04/msg05982.html Changes from v4: - Introduce spapr-nvdimm device with nvdimm device as the parent. - The new spapr-nvdimm has no new properties. As this is a new device and there is no migration related dependencies to be taken care of, the device behavior is made to set the device tree property and enable hcall when the device type spapr-nvdimm is used with pmem=off - Fixed commit messages - Added checks to ensure the backend is actualy file and not memory - Addressed things pointed out by Eric 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): nvdimm: Add realize, unrealize callbacks to NVDIMMDevice class spapr: nvdimm: Implement H_SCM_FLUSH hcall spapr: nvdimm: Introduce spapr-nvdimm device hw/mem/nvdimm.c | 16 ++ hw/mem/pc-dimm.c | 5 + hw/ppc/spapr.c | 2 + hw/ppc/spapr_nvdimm.c | 394 ++++++++++++++++++++++++++++++++++ include/hw/mem/nvdimm.h | 2 + include/hw/mem/pc-dimm.h | 1 + include/hw/ppc/spapr.h | 4 +- include/hw/ppc/spapr_nvdimm.h | 1 + 8 files changed, 424 insertions(+), 1 deletion(-) -- Signature
