On Thu, Jan 24, 2019 at 07:21:03PM +0800, Yi Zhang wrote: > 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.
This is where compatibility is being broken, isn't it? People currently using pmem=on on a regular file will start getting errors after a QEMU upgrade. Existing VMs with pmem=on may stop booting. Maybe this is OK, but we need to be able to explain why it is OK. > > 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. I'm trying to find out what you need the code to do, and the 3 items above are possible use cases that we might need to support. I'm not claiming we need to support all of them, but I would like to understand which ones you need to support. Once we answer that, we can choose what's the command-line required for each case. Right now this is not clear. > > > > ] 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) I don't get this: you seem to be saying your series implement (2), but above you say that users will get an error if using pmem=on on a filesystem not supporting DAX, which means MAP_SYNC is required but not optional (3). In either case, the choice between (1), (2) or (3) must depend only on command-line options, not on the QEMU build environment. "pmem=on" must always mean the same thing. If pmem=on is documented as making MAP_SYNC required (not optional), it should make MAP_SYNC required every time. If pmem=on is documented as making MAP_SYNC desired but optional, it should make MAP_SYNC optional every time. If you want pmem=on to mean something else not listed above, it may be also OK, as long as the meaning of pmem=on doesn't depend on the build time environment. With the current version of the series, the user can't be sure if pmem=on will enable MAP_SYNC or not, because its meaning depends on the version of the headers when QEMU was compiled. > 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 -- Eduardo
