[Qemu-devel] [PATCH V9 6/6] docs: Added MAP_SYNC documentation

[Zhang Yi]

Signed-off-by: Zhang Yi <address@hidden>
---
 docs/nvdimm.txt | 21 ++++++++++++++++++++-
 qemu-options.hx |  4 ++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
index 5f158a6..565ba73 100644
--- a/docs/nvdimm.txt
+++ b/docs/nvdimm.txt
@@ -142,11 +142,30 @@ backend of vNVDIMM:
 Guest Data Persistence
 ----------------------
 
+vNVDIMM is designed and implemented to guarantee the guest data
+persistence on the backends even on the host crash and power
+failures. However, there are still some requirements and limitations
+as explained below.
+
 Though QEMU supports multiple types of vNVDIMM backends on Linux,
-currently the only one that can guarantee the guest write persistence
+if MAP_SYNC is not supported by the host kernel and the backends,
+the only backend 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.
 
+mmap(2) flag MAP_SYNC is added since Linux kernel 4.15. On such
+systems, QEMU can mmap(2) the backend with MAP_SYNC, which can ensure
+filesystem metadata consistent even after a system crash or power
+failure. Besides the host kernel support, enabling MAP_SYNC in QEMU
+also requires:
+
+ - the backend is a file supporting DAX, e.g., a file on an ext4 or
+   xfs file system mounted with '-o dax',
+
+ - 'share' option of memory-backend-file is 'on'.
+
+ - 'pmem' option of memory-backend-file is 'on'
+
 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
 guest NVDIMM region mapping structure.  This unarmed flag indicates
diff --git a/qemu-options.hx b/qemu-options.hx
index 08f8516..545cb8a 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4002,6 +4002,10 @@ using the SNIA NVM programming model (e.g. Intel NVDIMM).
 If @option{pmem} is set to 'on', QEMU will take necessary operations to
 guarantee the persistence of its own writes to @option{mem-path}
 (e.g. in vNVDIMM label emulation and live migration).
+Also, we will map the backend-file with MAP_SYNC flag, which can ensure
+the file metadata is in sync to @option{mem-path} even on the host crash
+and power failures. MAP_SYNC requires supports from both the host kernel
+(since Linux kernel 4.15) and @option{mem-path} (only files supporting DAX).
 
 @item -object 
memory-backend-ram,address@hidden,address@hidden|off},address@hidden|off},address@hidden|off},address@hidden|off},address@hidden,address@hidden,address@hidden|preferred|bind|interleave}
 
-- 
2.7.4