[PATCH] docs/nvdimm: add 'pmem=on' for the device dax backend file

[Jingqi Liu <jingqi.liu@intel.com>]

At the end of live migration, QEMU uses msync() to flush the data to
the backend storage. When the backend file is a character device dax,
the pages explicitly avoid the page cache. It will return failure from msync().
The following warning is output.

    "warning: qemu_ram_msync: failed to sync memory range“

So we add 'pmem=on' to avoid calling msync(), use the QEMU command line:

    -object memory-backend-file,id=mem1,pmem=on,mem-path=/dev/dax0.0,size=4G

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jingqi Liu <jingqi.liu@intel.com>
---
 docs/nvdimm.txt | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
index c2c6e441b3..31048aff5e 100644
--- a/docs/nvdimm.txt
+++ b/docs/nvdimm.txt
@@ -243,6 +243,13 @@ use the QEMU command line:
 
     -object memory-backend-file,id=nv_mem,mem-path=/XXX/yyy,size=4G,pmem=on
 
+At the end of live migration, QEMU uses msync() to flush the data to the
+backend storage. When the backend file is a character device dax, the pages
+explicitly avoid the page cache. It will return failure from msync().
+So we add 'pmem=on' to avoid calling msync(), use the QEMU command line:
+
+    -object memory-backend-file,id=mem1,pmem=on,mem-path=/dev/dax0.0,size=4G
+
 References
 ----------
 
-- 
2.17.1