On Tue, Dec 18, 2018 at 04:17:39PM +0800, Zhang Yi wrote: > This option controls whether QEMU mmap(2)
will mmap > the memory backend file with > MAP_SYNC flag, which could consistent filesystem metadata I'm not sure what does "which could consistent" above mean. > for each guest > write, if MAP_SYNC flag is supported by the host kernel(Linux kernel 4.15 space before ( here pls. > and later) and the backend is a file supporting DAX (e.g., file on ext4/xfs > file system mounted with '-o dax'). > > It can take one of following values: > - on: try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or > 'share=off' or 'pmem!=on', QEMU will abort Really abort? What if it's added with object_add? Should just fail not abort in that case ... Also you know whether it's built into qemu at build time - how about skipping it in the schema and man page in that case? > - off: never pass MAP_SYNC to mmap(2) > - auto (default): if MAP_SYNC is supported and 'share=on' 'pmem=on', work as > if 'sync=on'; otherwise, work as if 'sync=off' So this is a mechanism. And users really do not care about the mechanism. What do users want? See discussion in nvdimm.txt > Signed-off-by: Haozhong Zhang <haozhong.zh...@intel.com> > Signed-off-by: Zhang Yi <yi.z.zh...@linux.intel.com> > --- > backends/hostmem-file.c | 39 +++++++++++++++++++++++++++++++++++++++ > docs/nvdimm.txt | 22 +++++++++++++++++++++- > include/exec/memory.h | 8 ++++++++ > qemu-options.hx | 22 +++++++++++++++++++++- > 4 files changed, 89 insertions(+), 2 deletions(-) > > diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c > index 0dd7a90..73cf181 100644 > --- a/backends/hostmem-file.c > +++ b/backends/hostmem-file.c > @@ -16,6 +16,7 @@ > #include "sysemu/hostmem.h" > #include "sysemu/sysemu.h" > #include "qom/object_interfaces.h" > +#include "qapi/qapi-visit.h" > > /* hostmem-file.c */ > /** > @@ -36,6 +37,7 @@ struct HostMemoryBackendFile { > uint64_t align; > bool discard_data; > bool is_pmem; > + OnOffAuto sync; > }; > > static void > @@ -62,6 +64,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error > **errp) > path, > backend->size, fb->align, > (backend->share ? RAM_SHARED : 0) | > + qemu_ram_sync_flags(fb->sync) | > (fb->is_pmem ? RAM_PMEM : 0), > fb->mem_path, errp); > g_free(path); > @@ -136,6 +139,39 @@ static void file_memory_backend_set_align(Object *o, > Visitor *v, > error_propagate(errp, local_err); > } > > +static void file_memory_backend_get_sync( > + Object *obj, Visitor *v, const char *name, void *opaque, Error **errp) > +{ > + HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(obj); > + OnOffAuto value = fb->sync; > + > + visit_type_OnOffAuto(v, name, &value, errp); > +} > + > +static void file_memory_backend_set_sync( > + Object *obj, Visitor *v, const char *name, void *opaque, Error **errp) > +{ > + HostMemoryBackend *backend = MEMORY_BACKEND(obj); > + HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(obj); > + Error *local_err = NULL; > + OnOffAuto value; > + > + if (host_memory_backend_mr_inited(backend)) { > + error_setg(&local_err, "cannot change property '%s' of %s", > + name, object_get_typename(obj)); > + goto out; > + } > + > + visit_type_OnOffAuto(v, name, &value, &local_err); > + if (local_err) { > + goto out; > + } > + fb->sync = value; > + > + out: > + error_propagate(errp, local_err); > +} > + > static bool file_memory_backend_get_pmem(Object *o, Error **errp) > { > return MEMORY_BACKEND_FILE(o)->is_pmem; > @@ -203,6 +239,9 @@ file_backend_class_init(ObjectClass *oc, void *data) > object_class_property_add_bool(oc, "pmem", > file_memory_backend_get_pmem, file_memory_backend_set_pmem, > &error_abort); > + object_class_property_add(oc, "sync", "OnOffAuto", > + file_memory_backend_get_sync, file_memory_backend_set_sync, > + NULL, NULL, &error_abort); > } > > static void file_backend_instance_finalize(Object *o) > diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt > index 5f158a6..d86a270 100644 > --- a/docs/nvdimm.txt > +++ b/docs/nvdimm.txt > @@ -142,11 +142,31 @@ backend of vNVDIMM: > Guest Data Persistence > ---------------------- > > +vNVDIMM is designed and implemented to guarantee > the guest data > +persistence on the backends even on the host crash and power > +failures. However, there are still some requirements and limitations > +as explained below. > + How about: nNVDIMM can guarantee guest data persistence in case of a host failure such as a crash or power failure, but only if the following conditions are satistied. > 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. BTW I think this user-facing detail belongs in the man page, not the source doc. > > +mmap(2) flag MAP_SYNC is added since Linux kernel 4.15. On such > +systems, QEMU can mmap(2) the backend with MAP_SYNC, which could Please check all your uses of "could" and "may" and replace with stronger wording that matches reality. I am guessing it's copied from some other language and I'm trying not to be a language purist but it's confusing in English. E.g. no one wants an implementation that could guarantee consistence - people want one that does guarantee it. > +consistent filesystem metadata for each guest write. Besides the host > +kernel support, enabling MAP_SYNC in QEMU also requires: > + > + - the backend is a file supporting DAX, e.g., a file on an ext4 or > + xfs file system mounted with '-o dax', Why list the filesystems here? The list can change with Linux. > + > + - 'sync' option of memory-backend-file is not 'off', and > + > + - 'share' option of memory-backend-file is 'on'. > + > + - 'pmem' option of memory-backend-file is 'on' > + Wait isn't this what pmem was supposed to do? Doesn't it mean "persistent memory"? Just goes to show that abbreviation is not good in a user interface IMHO. I think a higher level name like e.g. "host-failure-persistent" would be better. Have that set whatever flags you need to actually be persistent. Thoughts? > 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/include/exec/memory.h b/include/exec/memory.h > index c74c467..b398abb 100644 > --- a/include/exec/memory.h > +++ b/include/exec/memory.h > @@ -26,6 +26,7 @@ > #include "qom/object.h" > #include "qemu/rcu.h" > #include "hw/qdev-core.h" > +#include "qapi/error.h" > > #define RAM_ADDR_INVALID (~(ram_addr_t)0) > > @@ -136,6 +137,13 @@ typedef struct IOMMUNotifier IOMMUNotifier; > > #define RAM_SYNC (RAM_SYNC_ON_OFF_AUTO_ON | RAM_SYNC_ON_OFF_AUTO_AUTO) > > +static inline uint64_t qemu_ram_sync_flags(OnOffAuto v) > +{ > + return v == ON_OFF_AUTO_OFF ? RAM_SYNC_ON_OFF_AUTO_OFF : > + v == ON_OFF_AUTO_ON ? RAM_SYNC_ON_OFF_AUTO_ON : > + RAM_SYNC_ON_OFF_AUTO_AUTO; > +} > + That's pretty confusing. ON_OFF_AUTO_AUTO is painful enough. I don't think you want to duplicate this with all prefixes like RAM_SYNC_ON_OFF_AUTO_ON. Pls avoid doing it. > static inline void iommu_notifier_init(IOMMUNotifier *n, IOMMUNotify fn, > IOMMUNotifierFlag flags, > hwaddr start, hwaddr end, > diff --git a/qemu-options.hx b/qemu-options.hx > index 08f8516..624f634 100644 > --- a/qemu-options.hx > +++ b/qemu-options.hx > @@ -3928,7 +3928,7 @@ property must be set. These objects are placed in the > > @table @option > > -@item -object > memory-backend-file,id=@var{id},size=@var{size},mem-path=@var{dir},share=@var{on|off},discard-data=@var{on|off},merge=@var{on|off},dump=@var{on|off},prealloc=@var{on|off},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},align=@var{align} > +@item -object > memory-backend-file,id=@var{id},size=@var{size},mem-path=@var{dir},share=@var{on|off},discard-data=@var{on|off},merge=@var{on|off},dump=@var{on|off},prealloc=@var{on|off},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},align=@var{align},sync=@var{on|off|auto} > > Creates a memory file backend object, which can be used to back > the guest RAM with huge pages. > @@ -4003,6 +4003,26 @@ 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). > > +The @option{sync} option specifies whether QEMU mmap(2) @option{mem-path} > +with MAP_SYNC flag, which can guarantee the guest write persistence to > +@option{mem-path} even on the host crash and power failures. MAP_SYNC > +requires supports from both the host kernel (since Linux kernel 4.15) > +and @option{mem-path} (only files supporting DAX). It can take one of > +following values: > + > +@table @option > +@item @var{on} > +try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or > +@option{share}=@var{off}, @option{pmem}=@var{off} QEMU will abort > + > +@item @var{off} > +never pass MAP_SYNC to mmap(2) > + > +@item @var{auto} (default) > +if MAP_SYNC is supported and @option{share}=@var{on}, @option{pmem}=@var{on} > +work as if @option{sync}=@var{on}; otherwise, work as if > @option{sync}=@var{off} > +@end table > + > @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} > > Creates a memory backend object, which can be used to back the guest RAM. > -- > 2.7.4