hostmem: add property to map memory with MAP_SHARED

A new "share" property can be used with the "memory-file" backend to
map memory with MAP_SHARED instead of MAP_PRIVATE.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

backends/hostmem-file.c

@@ -28,6 +28,8 @@ typedef struct HostMemoryBackendFile HostMemoryBackendFile;
 
 struct HostMemoryBackendFile {
     HostMemoryBackend parent_obj;
+
+    bool share;
     char *mem_path;
 };
 
@@ -51,7 +53,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
         backend->force_prealloc = mem_prealloc;
         memory_region_init_ram_from_file(&backend->mr, OBJECT(backend),
                                  object_get_canonical_path(OBJECT(backend)),
-                                 backend->size,
+                                 backend->size, fb->share,
                                  fb->mem_path, errp);
     }
 #endif
@@ -87,9 +89,31 @@ static void set_mem_path(Object *o, const char *str, Error **errp)
     fb->mem_path = g_strdup(str);
 }
 
+static bool file_memory_backend_get_share(Object *o, Error **errp)
+{
+    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
+
+    return fb->share;
+}
+
+static void file_memory_backend_set_share(Object *o, bool value, Error **errp)
+{
+    HostMemoryBackend *backend = MEMORY_BACKEND(o);
+    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
+
+    if (memory_region_size(&backend->mr)) {
+        error_setg(errp, "cannot change property value");
+        return;
+    }
+    fb->share = value;
+}
+
 static void
 file_backend_instance_init(Object *o)
 {
+    object_property_add_bool(o, "share",
+                        file_memory_backend_get_share,
+                        file_memory_backend_set_share, NULL);
     object_property_add_str(o, "mem-path", get_mem_path,
                             set_mem_path, NULL);
 }

exec.c

@@ -73,6 +73,9 @@ static MemoryRegion io_mem_unassigned;
 /* RAM is pre-allocated and passed into qemu_ram_alloc_from_ptr */
 #define RAM_PREALLOC   (1 << 0)
 
+/* RAM is mmap-ed with MAP_SHARED */
+#define RAM_SHARED     (1 << 1)
+
 #endif
 
 struct CPUTailQ cpus = QTAILQ_HEAD_INITIALIZER(cpus);
@@ -1074,7 +1077,9 @@ static void *file_ram_alloc(RAMBlock *block,
         perror("ftruncate");
     }
 
-    area = mmap(0, memory, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
+    area = mmap(0, memory, PROT_READ | PROT_WRITE,
+                (block->flags & RAM_SHARED ? MAP_SHARED : MAP_PRIVATE),
+                fd, 0);
     if (area == MAP_FAILED) {
         error_setg_errno(errp, errno,
                          "unable to map backing store for hugepages");
@@ -1286,7 +1291,7 @@ static ram_addr_t ram_block_add(RAMBlock *new_block)
 
 #ifdef __linux__
 ram_addr_t qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
-                                    const char *mem_path,
+                                    bool share, const char *mem_path,
                                     Error **errp)
 {
     RAMBlock *new_block;
@@ -1311,6 +1316,7 @@ ram_addr_t qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
     new_block = g_malloc0(sizeof(*new_block));
     new_block->mr = mr;
     new_block->length = size;
+    new_block->flags = share ? RAM_SHARED : 0;
     new_block->host = file_ram_alloc(new_block, size,
                                      mem_path, errp);
     if (!new_block->host) {
@@ -1413,12 +1419,8 @@ void qemu_ram_remap(ram_addr_t addr, ram_addr_t length)
                 flags = MAP_FIXED;
                 munmap(vaddr, length);
                 if (block->fd >= 0) {
-#ifdef MAP_POPULATE
-                    flags |= mem_prealloc ? MAP_POPULATE | MAP_SHARED :
-                        MAP_PRIVATE;
-#else
-                    flags |= MAP_PRIVATE;
-#endif
+                    flags |= (block->flags & RAM_SHARED ?
+                              MAP_SHARED : MAP_PRIVATE);
                     area = mmap(vaddr, length, PROT_READ | PROT_WRITE,
                                 flags, block->fd, offset);
                 } else {

include/exec/memory.h

@@ -321,13 +321,15 @@ void memory_region_init_ram(MemoryRegion *mr,
  * @owner: the object that tracks the region's reference count
  * @name: the name of the region.
  * @size: size of the region.
+ * @share: %true if memory must be mmaped with the MAP_SHARED flag
  * @path: the path in which to allocate the RAM.
  * @errp: pointer to Error*, to store an error if it happens.
  */
 void memory_region_init_ram_from_file(MemoryRegion *mr,
                                       struct Object *owner,
                                       const char *name,
                                       uint64_t size,
+                                      bool share,
                                       const char *path,
                                       Error **errp);
 #endif

include/exec/ram_addr.h

@@ -23,7 +23,8 @@
 #include "hw/xen/xen.h"
 
 ram_addr_t qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
-                                    const char *mem_path, Error **errp);
+                                    bool share, const char *mem_path,
+                                    Error **errp);
 ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
                                    MemoryRegion *mr);
 ram_addr_t qemu_ram_alloc(ram_addr_t size, MemoryRegion *mr);

memory.c

@@ -1038,14 +1038,15 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
                                       struct Object *owner,
                                       const char *name,
                                       uint64_t size,
+                                      bool share,
                                       const char *path,
                                       Error **errp)
 {
     memory_region_init(mr, owner, name, size);
     mr->ram = true;
     mr->terminates = true;
     mr->destructor = memory_region_destructor_ram;
-    mr->ram_addr = qemu_ram_alloc_from_file(size, mr, path, errp);
+    mr->ram_addr = qemu_ram_alloc_from_file(size, mr, share, path, errp);
 }
 #endif
 

numa.c

@@ -231,7 +231,7 @@ static void allocate_system_memory_nonnuma(MemoryRegion *mr, Object *owner,
     if (mem_path) {
 #ifdef __linux__
         Error *err = NULL;
-        memory_region_init_ram_from_file(mr, owner, name, ram_size,
+        memory_region_init_ram_from_file(mr, owner, name, ram_size, false,
                                          mem_path, &err);
 
         /* Legacy behavior: if allocation failed, fall back to