On Thu, Jan 24, 2019 at 02:05:45PM -0500, Michael S. Tsirkin wrote: > On Thu, Jan 24, 2019 at 04:28:39PM -0200, Eduardo Habkost wrote: > > On Thu, Jan 24, 2019 at 12:45:54PM -0500, Michael S. Tsirkin wrote: > > > On Thu, Jan 24, 2019 at 02:59:26PM -0200, Eduardo Habkost wrote: > > > > 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> > > [...] > > > > > > > + - '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. > > > > > > I think it's OK since pmem explicitly means "persistent": > > > > > > The @option{pmem} option specifies whether the backing file specified > > > by @option{mem-path} is in host persistent memory that can be accessed > > > 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). > > > > If it's OK, let's at least explicitly document that we are > > breaking compatibility in those cases. > > > > > > > > > > > [...] > > > I think generally MAP_SYNC is required. > > > But for compatibility reasons we might need to support > > > !MAP_SYNC on old kernels even though it's risky. > > > > What about making MAP_SYNC optional only on older machine-types? > > I don't think this makes sense. It's not a guest visible change, > machine types are for that.
Losing data written to persistent memory is surely guest-visible behavior. -- Eduardo
