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 5f158a61708e1af6ea1912f796bc4f513ce1c1e9..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