Merge remote-tracking branch 'remotes/bonzini/tags/for-upstream' into staging

[mirror_qemu.git] / docs / nvdimm.txt

diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
index 24b443b655c4c7272fa5eaf93c85d4cf7ba01441..b531cacd35f0c8066f0247e74a283b2742f74383 100644 (file)
--- a/docs/nvdimm.txt
+++ b/docs/nvdimm.txt
@@ -49,8 +49,9 @@ Multiple vNVDIMM devices can be created if multiple pairs of "-object"
 and "-device" are provided.
 
 For above command line options, if the guest OS has the proper NVDIMM
-driver, it should be able to detect a NVDIMM device which is in the
-persistent memory mode and whose size is $NVDIMM_SIZE.
+driver (e.g. "CONFIG_ACPI_NFIT=y" under Linux), it should be able to
+detect a NVDIMM device which is in the persistent memory mode and whose
+size is $NVDIMM_SIZE.
 
 Note:
 
@@ -143,9 +144,25 @@ Guest Data Persistence
 ----------------------
 
 Though QEMU supports multiple types of vNVDIMM backends on Linux,
-currently the only one that can guarantee the guest write persistence
-is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to
-which all guest access do not involve any host-side kernel cache.
+the only backend that can guarantee the guest write persistence is:
+
+A. DAX device (e.g., /dev/dax0.0, ) or
+B. DAX file(mounted with dax option)
+
+When using B (A file supporting direct mapping of persistent memory)
+as a backend, write persistence is guaranteed if the host kernel has
+support for the MAP_SYNC flag in the mmap system call (available
+since Linux 4.15 and on certain distro kernels) and additionally
+both 'pmem' and 'share' flags are set to 'on' on the backend.
+
+If these conditions are not satisfied i.e. if either 'pmem' or 'share'
+are not set, if the backend file does not support DAX or if MAP_SYNC
+is not supported by the host kernel, write persistence is not
+guaranteed after a system crash. For compatibility reasons, these
+conditions are ignored if not satisfied. Currently, no way is
+provided to test for them.
+For more details, please reference mmap(2) man page:
+http://man7.org/linux/man-pages/man2/mmap.2.html.
 
 When using other types of backends, it's suggested to set 'unarmed'
 option of '-device nvdimm' to 'on', which sets the unarmed flag of the
@@ -173,3 +190,25 @@ There are currently two valid values for this option:
              the NVDIMMs in the event of power loss.  This implies that the
              platform also supports flushing dirty data through the memory
              controller on power loss.
+
+If the vNVDIMM backend is in host persistent memory that can be accessed in
+SNIA NVM Programming Model [1] (e.g., Intel NVDIMM), it's suggested to set
+the 'pmem' option of memory-backend-file to 'on'. When 'pmem' is 'on' and QEMU
+is built with libpmem [2] support (configured with --enable-libpmem), QEMU
+will take necessary operations to guarantee the persistence of its own writes
+to the vNVDIMM backend(e.g., in vNVDIMM label emulation and live migration).
+If 'pmem' is 'on' while there is no libpmem support, qemu will exit and report
+a "lack of libpmem support" message to ensure the persistence is available.
+For example, if we want to ensure the persistence for some backend file,
+use the QEMU command line:
+
+    -object memory-backend-file,id=nv_mem,mem-path=/XXX/yyy,size=4G,pmem=on
+
+References
+----------
+
+[1] NVM Programming Model (NPM)
+       Version 1.2
+    https://www.snia.org/sites/default/files/technical_work/final/NVMProgrammingModel_v1.2.pdf
+[2] Persistent Memory Development Kit (PMDK), formerly known as NVML project, home page:
+    http://pmem.io/pmdk/