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: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/block.c b/block.c
index 16436670dc..9b3cb75646 100644
--- a/block.c
+++ b/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);