block: add interface to toggle copy-on-read

The bdrv_enable_copy_on_read()/bdrv_disable_copy_on_read() functions can
be used to programmatically enable or disable copy-on-read for a block
device.  Later patches add the actual copy-on-read logic.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -538,6 +538,22 @@ int bdrv_parse_cache_flags(const char *mode, int *flags)
     return 0;
 }
 
+/**
+ * The copy-on-read flag is actually a reference count so multiple users may
+ * use the feature without worrying about clobbering its previous state.
+ * Copy-on-read stays enabled until all users have called to disable it.
+ */
+void bdrv_enable_copy_on_read(BlockDriverState *bs)
+{
+    bs->copy_on_read++;
+}
+
+void bdrv_disable_copy_on_read(BlockDriverState *bs)
+{
+    assert(bs->copy_on_read > 0);
+    bs->copy_on_read--;
+}
+
 /*
  * Common part for opening disk images and files
  */
@@ -559,6 +575,11 @@ static int bdrv_open_common(BlockDriverState *bs, const char *filename,
     bs->growable = 0;
     bs->buffer_alignment = 512;
 
+    assert(bs->copy_on_read == 0); /* bdrv_new() and bdrv_close() make it so */
+    if ((flags & BDRV_O_RDWR) && (flags & BDRV_O_COPY_ON_READ)) {
+        bdrv_enable_copy_on_read(bs);
+    }
+
     pstrcpy(bs->filename, sizeof(bs->filename), filename);
     bs->backing_file[0] = '\0';
 
@@ -801,6 +822,7 @@ void bdrv_close(BlockDriverState *bs)
 #endif
         bs->opaque = NULL;
         bs->drv = NULL;
+        bs->copy_on_read = 0;
 
         if (bs->file != NULL) {
             bdrv_close(bs->file);

block.h

@@ -70,6 +70,7 @@ typedef struct BlockDevOps {
 #define BDRV_O_NATIVE_AIO  0x0080 /* use native AIO instead of the thread pool */
 #define BDRV_O_NO_BACKING  0x0100 /* don't open the backing file */
 #define BDRV_O_NO_FLUSH    0x0200 /* disable flushing on this disk */
+#define BDRV_O_COPY_ON_READ 0x0400 /* copy read backing sectors into image */
 
 #define BDRV_O_CACHE_MASK  (BDRV_O_NOCACHE | BDRV_O_CACHE_WB | BDRV_O_NO_FLUSH)
 
@@ -312,6 +313,9 @@ void bdrv_reset_dirty(BlockDriverState *bs, int64_t cur_sector,
                       int nr_sectors);
 int64_t bdrv_get_dirty_count(BlockDriverState *bs);
 
+void bdrv_enable_copy_on_read(BlockDriverState *bs);
+void bdrv_disable_copy_on_read(BlockDriverState *bs);
+
 void bdrv_set_in_use(BlockDriverState *bs, int in_use);
 int bdrv_in_use(BlockDriverState *bs);
 

block_int.h

@@ -198,6 +198,8 @@ struct BlockDriverState {
     int encrypted; /* if true, the media is encrypted */
     int valid_key; /* if true, a valid encryption key has been set */
     int sg;        /* if true, the device is a /dev/sg* */
+    int copy_on_read; /* if true, copy read backing sectors into image
+                         note this is a reference count */
 
     BlockDriver *drv; /* NULL means no media */
     void *opaque;