On 2019-01-23 at 12:50:50 -0200, Eduardo Habkost wrote: > On Wed, Jan 23, 2019 at 11:00:02AM +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 | 29 ++++++++++++++++++++++++++++- > > qemu-options.hx | 4 ++++ > > 2 files changed, 32 insertions(+), 1 deletion(-) > > > > diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt > > index 5f158a6..166c395 100644 > > --- a/docs/nvdimm.txt > > +++ b/docs/nvdimm.txt > > @@ -142,11 +142,38 @@ backend of vNVDIMM: > > Guest Data Persistence > > ---------------------- > > > > +vNVDIMM is designed and implemented to guarantee the guest data > > +persistence on the backends in case of host crash or a 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 dax backend files with MAP_SYNC, which > > +ensures filesystem metadata consistency in case of a host crash or a power > > +failure. Enabling MAP_SYNC in QEMU requires below conditions > > + > > + - 'pmem' option of memory-backend-file is 'on': > > + The backend is a file supporting DAX, e.g., a file on an ext4 or > > + xfs file system mounted with '-o dax'. if your pmem=on ,but the backend > > is > > + not a file supporting DAX, mapping with this flag results in an > > EOPNOTSUPP > > + error. > > Won't this break existing configurations that work today on QEMU > 3.1.0? Why exactly it is OK to break compatibility here? won't, pmem option default is off, if people who start VM don't know what backend file is, it is suggested and *default to set pmem=off, if people well know the backend file have dax capbility. it is suggest to set pmem=on.
For a special case that we use /dev/dax as backend, we already have a patch to add MAP_SYNC falg mapiing from device dax mode. see https://lkml.org/lkml/2018/4/22/524 So, if people force set pmem=on, mapping a regular file, it will results in an EOPNOTSUPP error. see http://man7.org/linux/man-pages/man2/mmap.2.html > > > + > > + - 'share' option of memory-backend-file is 'on': > > + MAP_SYNC flag available only with the MAP_SHARED_VALIDATE mapping type. > > I don't understand what this paragraph means. see http://man7.org/linux/man-pages/man2/mmap.2.html > > > + > > + - 'MAP_SYNC' is supported on linux kernel.(default opened since Linux > > 4.15) > > + > > I don't understand why you are making the semantics of > command-line options change depending on the host kernel. the option pmem=on do not dependent the host kernel. MAP_SYNC will be ignore if the kernel don't support. the "pmem=on" have another meaning see https://patchwork.kernel.org/patch/10459407/ > > > +Otherwise, We will ignore the MAP_SYNC flag. > > + > > See the questions I sent about supported use cases at > <https://www.mail-archive.com/qemu-devel@nongnu.org/msg588822.html>. > I still don't see those questions answered: > > ] We have at least 3 different possible use cases we might need to > ] support: > ] > ] 1) pmem=on, MAP_SYNC not desired > ] 2) pmem=on, MAP_SYNC desired but optional > ] 3) pmem=on, MAP_SYNC required, not optional > ] Sorry for my poor understanding, I don't know what these mean? pmem=on will force flag the MAP_SYNC while it capable on current kernel. As we talk with Micheal if we set pmem=on , MAP_SYNC is always desired. Means, if pmem=on, there is no option to close MAP_SYNC seprately. > ] Which cases from the list above we need to support? > ] > ] From the cases above, what's the expected semantics of "pmem=on" > ] with no extra options? > > It's not clear to me yet if you want to support use cases (1) and (2). > > Also, you seem to be choosing between use case (1) or (3) depending on > the build environment instead of command-line options. The > meaning of command-line options must be predictable and > unambiguous, and not depend on build time variables. so you are asking? 1) pmem=on, MAP_SYNC not supported kernel - MAP_SYNC will be defined 0 and will be ignored in this case. see 2/4. 2) pmem=on, MAP_SYNC is supported but have a option to pass to mmap2() - v7 send-out for a option sync to open/close MAP_SYNC seprately. After talking with Micheal, we give up on a bit of flexibility, and just say pmem=on forces MAP_SYNC. on a MAP_SYNC capable configrations(kernel+ backend dax) 3) pmem=on, ? > > > > +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 > > guest NVDIMM region mapping structure. This unarmed flag indicates > > diff --git a/qemu-options.hx b/qemu-options.hx > > index 08f8516..0cd41f4 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} in case of host crash > > +or a power failure. MAP_SYNC requires support from both the host kernel > > +(since Linux kernel 4.15) and @option{mem-path} (only files supporting > > DAX). > > > > @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 > > > > -- > Eduardo
