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[Qemu-devel] [PATCH V7 6/6] hostmem-file: add 'sync' option

From: Zhang Yi
Subject: [Qemu-devel] [PATCH V7 6/6] hostmem-file: add 'sync' option
Date: Tue, 18 Dec 2018 16:17:39 +0800

This option controls whether QEMU mmap(2) the memory backend file with
MAP_SYNC flag, which could consistent filesystem metadata for each guest
write, 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' or 'pmem!=on', QEMU will abort
 - 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'

Signed-off-by: Haozhong Zhang <address@hidden>
Signed-off-by: Zhang Yi <address@hidden>
 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 
                                  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);
@@ -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,
+    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.
 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 could
+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',
+ - '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'
 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;
+static inline uint64_t qemu_ram_sync_flags(OnOffAuto v)
+           v == ON_OFF_AUTO_ON ? RAM_SYNC_ON_OFF_AUTO_ON :
 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
address@hidden -object 
address@hidden -object 
 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
address@hidden 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:
address@hidden @option
address@hidden @var{on}
+try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
address@hidden@var{off}, @address@hidden QEMU will abort
address@hidden @var{off}
+never pass MAP_SYNC to mmap(2)
address@hidden @var{auto} (default)
+if MAP_SYNC is supported and @address@hidden, @address@hidden
+work as if @address@hidden; otherwise, work as if @address@hidden
address@hidden table
 @item -object 
 Creates a memory backend object, which can be used to back the guest RAM.

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