block: Add BDRV_BLOCK_EOF to bdrv_get_block_status()

Just as the block layer already sets BDRV_BLOCK_ALLOCATED as a
shortcut for subsequent operations, there are also some optimizations
that are made easier if we can quickly tell that *pnum will advance
us to the end of a file, via a new BDRV_BLOCK_EOF which gets set
by the block layer.

This just plumbs up the new bit; subsequent patches will make use
of it.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20170505021500.19315-2-eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

2 files changed, 13 insertions, 4 deletions

diff --git a/block/io.c b/block/io.c
index 9bba730..7e6abef 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1711,15 +1711,16 @@ typedef struct BdrvCoGetBlockStatusData {
  * Drivers not implementing the functionality are assumed to not support
  * backing files, hence all their sectors are reported as allocated.
  *
- * If 'sector_num' is beyond the end of the disk image the return value is 0
- * and 'pnum' is set to 0.
+ * If 'sector_num' is beyond the end of the disk image the return value is
+ * BDRV_BLOCK_EOF and 'pnum' is set to 0.
  *
  * 'pnum' is set to the number of sectors (including and immediately following
  * the specified sector) that are known to be in the same
  * allocated/unallocated state.
  *
  * 'nb_sectors' is the max value 'pnum' should be set to.  If nb_sectors goes
- * beyond the end of the disk image it will be clamped.
+ * beyond the end of the disk image it will be clamped; if 'pnum' is set to
+ * the end of the image, then the returned value will include BDRV_BLOCK_EOF.
  *
  * If returned value is positive and BDRV_BLOCK_OFFSET_VALID bit is set, 'file'
  * points to the BDS which the sector range is allocated in.
@@ -1740,7 +1741,7 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
 
     if (sector_num >= total_sectors) {
         *pnum = 0;
-        return 0;
+        return BDRV_BLOCK_EOF;
     }
 
     n = total_sectors - sector_num;
@@ -1751,6 +1752,9 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
     if (!bs->drv->bdrv_co_get_block_status) {
         *pnum = nb_sectors;
         ret = BDRV_BLOCK_DATA | BDRV_BLOCK_ALLOCATED;
+        if (sector_num + nb_sectors == total_sectors) {
+            ret |= BDRV_BLOCK_EOF;
+        }
         if (bs->drv->protocol_name) {
             ret |= BDRV_BLOCK_OFFSET_VALID | (sector_num * BDRV_SECTOR_SIZE);
         }
@@ -1814,6 +1818,9 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
 
 out:
     bdrv_dec_in_flight(bs);
+    if (ret >= 0 && sector_num + *pnum == total_sectors) {
+        ret |= BDRV_BLOCK_EOF;
+    }
     return ret;
 }
 
diff --git a/include/block/block.h b/include/block/block.h
index 85e4be7..4c149adb 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -129,6 +129,7 @@ typedef struct HDGeometry {
  * BDRV_BLOCK_OFFSET_VALID: an associated offset exists for accessing raw data
  * BDRV_BLOCK_ALLOCATED: the content of the block is determined by this
  *                       layer (short for DATA || ZERO), set by block layer
+ * BDRV_BLOCK_EOF: the returned pnum covers through end of file for this layer
  *
  * Internal flag:
  * BDRV_BLOCK_RAW: used internally to indicate that the request was
@@ -157,6 +158,7 @@ typedef struct HDGeometry {
 #define BDRV_BLOCK_OFFSET_VALID 0x04
 #define BDRV_BLOCK_RAW          0x08
 #define BDRV_BLOCK_ALLOCATED    0x10
+#define BDRV_BLOCK_EOF          0x20
 #define BDRV_BLOCK_OFFSET_MASK  BDRV_SECTOR_MASK
 
 typedef QSIMPLEQ_HEAD(BlockReopenQueue, BlockReopenQueueEntry) BlockReopenQueue;