9pfs: clarify latency of v9fs_co_run_in_worker()

As we just fixed a severe performance issue with Treaddir request
handling, clarify this overall issue as a comment on
v9fs_co_run_in_worker() with the intention to hopefully prevent
such performance mistakes in future (and fixing other yet
outstanding ones).

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <4d34d332e1aaa8a2cf8dc0b5da4fd7727f2a86e8.1596012787.git.qemu_oss@crudebyte.com>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

hw/9pfs/coth.h

@@ -19,7 +19,7 @@
 #include "qemu/coroutine.h"
 #include "9p.h"
 
-/*
+/**
  * we want to use bottom half because we want to make sure the below
  * sequence of events.
  *
@@ -28,6 +28,16 @@
  *   3. Enter the coroutine in the worker thread.
  * we cannot swap step 1 and 2, because that would imply worker thread
  * can enter coroutine while step1 is still running
+ *
+ * @b PERFORMANCE @b CONSIDERATIONS: As a rule of thumb, keep in mind
+ * that hopping between threads adds @b latency! So when handling a
+ * 9pfs request, avoid calling v9fs_co_run_in_worker() too often, because
+ * this might otherwise sum up to a significant, huge overall latency for
+ * providing the response for just a single request. For that reason it
+ * is highly recommended to fetch all data from fs driver with a single
+ * fs driver request on a background I/O thread (bottom half) in one rush
+ * first and then eventually assembling the final response from that data
+ * on main I/O thread (top half).
  */
 #define v9fs_co_run_in_worker(code_block)                               \
     do {                                                                \