vmdk: Only read cid from image file when opening

Previously cid of parent is parsed from image file for every IO request.
We already have L1/L2 cache and don't have assumption that parent image
can be updated behind us, so remove this to get more efficiency.

The parent CID is checked only for once after opening.
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/vmdk.c

@@ -112,6 +112,7 @@ typedef struct BDRVVmdkState {
     CoMutex lock;
     uint64_t desc_offset;
     bool cid_updated;
+    bool cid_checked;
     uint32_t parent_cid;
     int num_extents;
     /* Extent array with num_extents entries, ascend ordered by address */
@@ -197,8 +198,6 @@ static int vmdk_probe(const uint8_t *buf, int buf_size, const char *filename)
     }
 }
 
-#define CHECK_CID 1
-
 #define SECTOR_SIZE 512
 #define DESC_SIZE (20 * SECTOR_SIZE)    /* 20 sectors of 512 bytes each */
 #define BUF_SIZE 4096
@@ -301,19 +300,18 @@ static int vmdk_write_cid(BlockDriverState *bs, uint32_t cid)
 
 static int vmdk_is_cid_valid(BlockDriverState *bs)
 {
-#ifdef CHECK_CID
     BDRVVmdkState *s = bs->opaque;
     BlockDriverState *p_bs = bs->backing_hd;
     uint32_t cur_pcid;
 
-    if (p_bs) {
+    if (!s->cid_checked && p_bs) {
         cur_pcid = vmdk_read_cid(p_bs, 0);
         if (s->parent_cid != cur_pcid) {
             /* CID not valid */
             return 0;
         }
     }
-#endif
+    s->cid_checked = true;
     /* CID valid */
     return 1;
 }