[RFC PATCH v2 1/3] exec/memory: Introduce memory_region_init_rom_device_from_file()

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Introduce memory_region_init_rom_device_from_file() which mmap
the backing file of ROM devices. This allows to reduce QEMU memory
footprint as the same file can be shared between multiple instances
of QEMU.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 include/exec/memory.h | 85 +++++++++++++++++++++++++++++++++++++
 softmmu/memory.c      | 98 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 183 insertions(+)

diff --git a/include/exec/memory.h b/include/exec/memory.h
index c6fb714e499..bacf7495003 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -487,6 +487,9 @@ struct MemoryRegion {
     const char *name;
     unsigned ioeventfd_nb;
     MemoryRegionIoeventfd *ioeventfds;
+#ifndef CONFIG_POSIX
+    gchar *contents;
+#endif
 };
 
 struct IOMMUMemoryRegion {
@@ -1131,6 +1134,43 @@ void memory_region_init_rom_device_nomigrate(MemoryRegion *mr,
                                              uint64_t size,
                                              Error **errp);
 
+/**
+ * memory_region_init_rom_device_from_file_nomigrate:
+ * Initialize a ROM memory region from the specified backing file.
+ * Writes are handled via callbacks.
+ *
+ * Note that this function does not do anything to cause the data in the
+ * RAM side of the memory region to be migrated; that is the responsibility
+ * of the caller.
+ *
+ * @mr: the #MemoryRegion to be initialized.
+ * @owner: the object that tracks the region's reference count
+ * @ops: callbacks for write access handling (must not be NULL).
+ * @opaque: passed to the read and write callbacks of the @ops structure.
+ * @name: Region name, becomes part of RAMBlock name used in migration stream
+ *        must be unique within any device
+ * @size: size of the region.
+ * @ram_flags: specify the properties of the ram block, which can be one
+ *             or bit-or of following values
+ *             - RAM_SHARED: mmap the backing file or device with MAP_SHARED
+ *             - RAM_PMEM: the backend @mem_path is persistent memory
+ *             Other bits are ignored.
+ * @path: specify the backing file
+ * @readonly: true to open @path for reading, false for read/write.
+ * @errp: pointer to Error*, to store an error if it happens.
+ */
+void memory_region_init_rom_device_from_file_nomigrate(MemoryRegion *mr,
+                                                       Object *owner,
+                                                       const MemoryRegionOps *ops,
+                                                       void *opaque,
+                                                       const char *name,
+                                                       uint64_t size,
+                                                       uint64_t align,
+                                                       uint32_t ram_flags,
+                                                       const char *path,
+                                                       bool readonly,
+                                                       Error **errp);
+
 /**
  * memory_region_init_iommu: Initialize a memory region of a custom type
  * that translates addresses
@@ -1249,6 +1289,51 @@ void memory_region_init_rom_device(MemoryRegion *mr,
                                    Error **errp);
 
 
+/**
+ * memory_region_init_rom_device_from_file:
+ * Initialize a ROM memory region from the specified backing file.
+ * Writes are handled via callbacks.
+ *
+ * This function initializes a memory region backed by RAM for reads
+ * and callbacks for writes, and arranges for the RAM backing to
+ * be migrated (by calling vmstate_register_ram()
+ * if @owner is a DeviceState, or vmstate_register_ram_global() if
+ * @owner is NULL).
+ *
+ * TODO: Currently we restrict @owner to being either NULL (for
+ * global RAM regions with no owner) or devices, so that we can
+ * give the RAM block a unique name for migration purposes.
+ * We should lift this restriction and allow arbitrary Objects.
+ * If you pass a non-NULL non-device @owner then we will assert.
+ *
+ * @mr: the #MemoryRegion to be initialized.
+ * @owner: the object that tracks the region's reference count
+ * @ops: callbacks for write access handling (must not be NULL).
+ * @opaque: passed to the read and write callbacks of the @ops structure.
+ * @name: Region name, becomes part of RAMBlock name used in migration stream
+ *        must be unique within any device
+ * @size: size of the region.
+ * @ram_flags: specify the properties of the ram block, which can be one
+ *             or bit-or of following values
+ *             - RAM_SHARED: mmap the backing file or device with MAP_SHARED
+ *             - RAM_PMEM: the backend @mem_path is persistent memory
+ *             Other bits are ignored.
+ * @path: specify the backing file
+ * @readonly: true to open @path for reading, false for read/write.
+ * @errp: pointer to Error*, to store an error if it happens.
+ */
+void memory_region_init_rom_device_from_file(MemoryRegion *mr,
+                                             Object *owner,
+                                             const MemoryRegionOps *ops,
+                                             void *opaque,
+                                             const char *name,
+                                             uint64_t size,
+                                             uint64_t align,
+                                             uint32_t ram_flags,
+                                             const char *path,
+                                             bool readonly,
+                                             Error **errp);
+
 /**
  * memory_region_owner: get a memory region's owner.
  *
diff --git a/softmmu/memory.c b/softmmu/memory.c
index 874a8fccdee..ea1892a8cd6 100644
--- a/softmmu/memory.c
+++ b/softmmu/memory.c
@@ -1120,6 +1120,14 @@ static void memory_region_destructor_ram(MemoryRegion *mr)
     qemu_ram_free(mr->ram_block);
 }
 
+#ifndef CONFIG_POSIX
+static void memory_region_destructor_contents(MemoryRegion *mr)
+{
+    qemu_ram_free(mr->ram_block);
+    g_free(mr->contents);
+}
+#endif
+
 static bool memory_region_need_escape(char c)
 {
     return c == '/' || c == '[' || c == '\\' || c == ']';
@@ -1712,6 +1720,96 @@ void memory_region_init_rom_device_nomigrate(MemoryRegion *mr,
     }
 }
 
+void memory_region_init_rom_device_from_file_nomigrate(MemoryRegion *mr,
+                                                       Object *owner,
+                                                       const MemoryRegionOps *ops,
+                                                       void *opaque,
+                                                       const char *name,
+                                                       uint64_t size,
+                                                       uint64_t align,
+                                                       uint32_t ram_flags,
+                                                       const char *path,
+                                                       bool readonly,
+                                                       Error **errp)
+{
+    Error *err = NULL;
+
+    assert(ops);
+#ifdef CONFIG_POSIX
+    memory_region_init(mr, owner, name, size);
+    mr->opaque = opaque;
+    mr->ops = ops;
+    mr->rom_device = true;
+    mr->readonly = readonly;
+    mr->ram = true;
+    mr->align = align;
+    mr->terminates = true;
+    mr->destructor = memory_region_destructor_ram;
+    mr->ram_block = qemu_ram_alloc_from_file(size, mr, ram_flags, path,
+                                             readonly, &err);
+    if (err) {
+        mr->size = int128_zero();
+        object_unparent(OBJECT(mr));
+        error_propagate(errp, err);
+    }
+#else
+    g_autoptr(GError) gerr = NULL;
+    gsize len;
+
+    memory_region_init(mr, owner, name, size);
+    mr->ops = ops;
+    mr->opaque = opaque;
+    mr->terminates = true;
+    mr->rom_device = true;
+
+    if (!g_file_get_contents(path, &mr->contents, &len, &gerr)) {
+        error_setg(errp, "Unable to read '%s': %s", path, gerr->message);
+        return;
+    }
+    mr->destructor = memory_region_destructor_contents;
+    mr->contents = g_realloc(mr->contents, size);
+    mr->ram_block = qemu_ram_alloc_from_ptr(size, mr->contents, mr, &err);
+    if (err) {
+        mr->size = int128_zero();
+        object_unparent(OBJECT(mr));
+        error_propagate(errp, err);
+    }
+#endif
+}
+
+void memory_region_init_rom_device_from_file(MemoryRegion *mr,
+                                             Object *owner,
+                                             const MemoryRegionOps *ops,
+                                             void *opaque,
+                                             const char *name,
+                                             uint64_t size,
+                                             uint64_t align,
+                                             uint32_t ram_flags,
+                                             const char *path,
+                                             bool readonly,
+                                             Error **errp)
+{
+    DeviceState *owner_dev;
+    Error *err = NULL;
+
+    memory_region_init_rom_device_from_file_nomigrate(mr, owner, ops, opaque,
+                                                      name, size, align,
+                                                      ram_flags, path, readonly,
+                                                      &err);
+    if (err) {
+        error_propagate(errp, err);
+        return;
+    }
+    /* This will assert if owner is neither NULL nor a DeviceState.
+     * We only want the owner here for the purposes of defining a
+     * unique name for migration. TODO: Ideally we should implement
+     * a naming scheme for Objects which are not DeviceStates, in
+     * which case we can relax this restriction.
+     */
+    owner_dev = DEVICE(owner);
+    vmstate_register_ram(mr, owner_dev);
+}
+
 void memory_region_init_iommu(void *_iommu_mr,
                               size_t instance_size,
                               const char *mrtypename,
-- 
2.26.2