On Fri, Feb 08, 2019 at 06:11:23PM +0800, Zhang, Yi wrote: > From: Zhang Yi <yi.z.zh...@linux.intel.com> > > Signed-off-by: Zhang Yi <yi.z.zh...@linux.intel.com> > --- > docs/nvdimm.txt | 25 ++++++++++++++++++++++--- > qemu-options.hx | 5 +++++ > 2 files changed, 27 insertions(+), 3 deletions(-) > > diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt > index 5f158a6..a168429 100644 > --- a/docs/nvdimm.txt > +++ b/docs/nvdimm.txt > @@ -143,9 +143,28 @@ 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) > + > +both are use real NVDIMM device as backend, which supporting direct > +access for files(no page cache).
So I would just drop above two lines. How does one know that there's support for direct access? it's just a question of kernel right? And I don't think it matter wrt "real NVDIMM" - e.g. it could be vNVDIMM in a nested virt scenario. It will still survive host (i.e. L1 guest) crash. Besides, the above two lines aren't grammatical. > + > +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 silently 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 > diff --git a/qemu-options.hx b/qemu-options.hx > index 08f8516..ef1da8f 100644 > --- a/qemu-options.hx > +++ b/qemu-options.hx > @@ -4002,6 +4002,11 @@ 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 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. > > @item -object > memory-backend-ram,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave} > > -- > 2.7.4
