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X-Received-From: 134.134.136.20 Subject: [Qemu-devel] [PATCH v15 2/2] docs: Added MAP_SYNC documentation X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: pagupta@redhat.com, xiaoguangrong.eric@gmail.com, mst@redhat.com, yi.z.zhang@linux.intel.com, yu.c.zhang@linux.intel.com, richardw.yang@linux.intel.com, stefanha@redhat.com, imammedo@redhat.com, pbonzini@redhat.com, dan.j.williams@intel.com, ehabkost@redhat.com Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" Content-Type: text/plain; charset="utf-8" From: Zhang Yi Signed-off-by: Zhang Yi Signed-off-by: Wei Yang Reviewed-by: Stefan Hajnoczi Reviewed-by: Pankaj Gupta Reviewed-by: Michael S. Tsirkin --- docs/nvdimm.txt | 22 +++++++++++++++++++--- qemu-options.hx | 5 +++++ 2 files changed, 24 insertions(+), 3 deletions(-) diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt index 7231c2d78f..b531cacd35 100644 --- a/docs/nvdimm.txt +++ b/docs/nvdimm.txt @@ -144,9 +144,25 @@ Guest Data Persistence ---------------------- =20 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. =20 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 diff --git a/qemu-options.hx b/qemu-options.hx index 08749a3391..bdc74c0620 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -4233,6 +4233,11 @@ using the SNIA NVM programming model (e.g. Intel NVD= IMM). 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 ensures the +file metadata is in sync for @option{mem-path} in case of host crash +or a power failure. MAP_SYNC requires support from both the host kernel +(since Linux kernel 4.15) and the filesystem of @option{mem-path} mounted +with DAX option. =20 @item -object memory-backend-ram,id=3D@var{id},merge=3D@var{on|off},dump= =3D@var{on|off},share=3D@var{on|off},prealloc=3D@var{on|off},size=3D@var{si= ze},host-nodes=3D@var{host-nodes},policy=3D@var{default|preferred|bind|inte= rleave} =20 --=20 2.19.1