This option controls whether QEMU mmap(2) the memory backend file with MAP_SYNC flag, which can fully guarantee the guest write persistence to the backend, if MAP_SYNC flag is supported by the host kernel (Linux kernel 4.15 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', QEMU will abort - off: never pass MAP_SYNC to mmap(2) - auto (default): if MAP_SYNC is supported and 'share=on', work as if 'sync=on'; otherwise, work as if 'sync=off' Signed-off-by: Haozhong Zhang <haozhong.zh...@intel.com> Suggested-by: Eduardo Habkost <ehabk...@redhat.com> Reviewed-by: Michael S. Tsirkin <m...@redhat.com> --- backends/hostmem-file.c | 41 ++++++++++++++++++++++++++++++++++++++++- docs/nvdimm.txt | 20 +++++++++++++++++++- qemu-options.hx | 22 +++++++++++++++++++++- 3 files changed, 80 insertions(+), 3 deletions(-) diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c index df06b547a6..ade80d76f1 100644 --- a/backends/hostmem-file.c +++ b/backends/hostmem-file.c @@ -15,6 +15,7 @@ #include "sysemu/hostmem.h" #include "sysemu/sysemu.h" #include "qom/object_interfaces.h" +#include "qapi-visit.h" /* hostmem-file.c */ /** @@ -35,6 +36,7 @@ struct HostMemoryBackendFile { bool discard_data; char *mem_path; uint64_t align; + OnOffAuto sync; }; static void @@ -60,7 +62,8 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp) memory_region_init_ram_from_file(&backend->mr, OBJECT(backend), path, backend->size, fb->align, - fb->share ? QEMU_RAM_SHARE : 0, + (fb->share ? QEMU_RAM_SHARE : 0) | + qemu_ram_sync_flags(fb->sync), fb->mem_path, errp); g_free(path); } @@ -154,6 +157,39 @@ 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 '%s'", + name, object_get_typename(obj), backend->id); + 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 void file_backend_unparent(Object *obj) { HostMemoryBackend *backend = MEMORY_BACKEND(obj); @@ -188,6 +224,9 @@ file_backend_class_init(ObjectClass *oc, void *data) file_memory_backend_get_align, file_memory_backend_set_align, NULL, NULL, &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 e903d8bb09..5e9cee5f5e 100644 --- a/docs/nvdimm.txt +++ b/docs/nvdimm.txt @@ -142,11 +142,29 @@ 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. + 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 backend with MAP_SYNC, which can +guarantee the guest write persistence to vNVDIMM. 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', + + - 'sync' option of memory-backend-file is not 'off', and + + - 'share' option of memory-backend-file is 'on'. + 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 8ce427da78..4e8ec5ed97 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -3957,7 +3957,7 @@ 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. @@ -4017,6 +4017,26 @@ requires an alignment different than the default one used by QEMU, eg the device DAX /dev/dax0.0 requires 2M alignment rather than 4K. In such cases, users can specify the required alignment via this option. +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}, 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}, 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},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. -- 2.14.1