As it is not obvious for a block neophyte what means
the 'log' value, document it.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 include/block/block.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/block/block.h b/include/block/block.h
index 480e6b6837..6836876da1 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -86,8 +86,8 @@ typedef enum {
 } BdrvRequestFlags;
 
 typedef struct BlockSizes {
-    uint32_t phys;
-    uint32_t log;
+    uint32_t phys;  /* physical block size */
+    uint32_t log;   /* logical block size */
 } BlockSizes;
 
 typedef struct HDGeometry {
-- 
2.21.3