block: clarify the meaning of BDRV_O_NOCACHE

Index: qemu/block.c
===================================================================
@@ -439,13 +439,7 @@ static int bdrv_open_common(BlockDriverS
     bs->drv = drv;
     bs->opaque = qemu_mallocz(drv->instance_size);
 
-    /*
-     * Yes, BDRV_O_NOCACHE aka O_DIRECT means we have to present a
-     * write cache to the guest.  We do need the fdatasync to flush
-     * out transactions for block allocations, and we maybe have a
-     * volatile write cache in our backing device to deal with.
-     */
-    if (flags & (BDRV_O_CACHE_WB|BDRV_O_NOCACHE))
+    if (flags & BDRV_O_CACHE_WB)
         bs->enable_write_cache = 1;
 
     /*

Index: qemu/block/raw-posix.c
===================================================================
@@ -154,7 +154,7 @@ static int raw_open_common(BlockDriverSt
      * and O_DIRECT for no caching. */
     if ((bdrv_flags & BDRV_O_NOCACHE))
         s->open_flags |= O_DIRECT;
-    else if (!(bdrv_flags & BDRV_O_CACHE_WB))
+    if (!(bdrv_flags & BDRV_O_CACHE_WB))
         s->open_flags |= O_DSYNC;
 
     s->fd = -1;

Index: qemu/block/raw-win32.c
===================================================================
@@ -88,9 +88,9 @@ static int raw_open(BlockDriverState *bs
     }
 
     overlapped = FILE_ATTRIBUTE_NORMAL;
-    if ((flags & BDRV_O_NOCACHE))
-        overlapped |= FILE_FLAG_NO_BUFFERING | FILE_FLAG_WRITE_THROUGH;
-    else if (!(flags & BDRV_O_CACHE_WB))
+    if (flags & BDRV_O_NOCACHE)
+        overlapped |= FILE_FLAG_NO_BUFFERING;
+    if (!(flags & BDRV_O_CACHE_WB))
         overlapped |= FILE_FLAG_WRITE_THROUGH;
     s->hfile = CreateFile(filename, access_flags,
                           FILE_SHARE_READ, NULL,

@@ -349,9 +349,9 @@ static int hdev_open(BlockDriverState *b
     create_flags = OPEN_EXISTING;
 
     overlapped = FILE_ATTRIBUTE_NORMAL;
-    if ((flags & BDRV_O_NOCACHE))
-        overlapped |= FILE_FLAG_NO_BUFFERING | FILE_FLAG_WRITE_THROUGH;
-    else if (!(flags & BDRV_O_CACHE_WB))
+    if (flags & BDRV_O_NOCACHE)
+        overlapped |= FILE_FLAG_NO_BUFFERING;
+    if (!(flags & BDRV_O_CACHE_WB))
         overlapped |= FILE_FLAG_WRITE_THROUGH;
     s->hfile = CreateFile(filename, access_flags,
                           FILE_SHARE_READ, NULL,

Index: qemu/blockdev.c
===================================================================
@@ -326,7 +326,7 @@ DriveInfo *drive_init(QemuOpts *opts, in
 
     if ((buf = qemu_opt_get(opts, "cache")) != NULL) {
         if (!strcmp(buf, "off") || !strcmp(buf, "none")) {
-            bdrv_flags |= BDRV_O_NOCACHE;
+            bdrv_flags |= BDRV_O_NOCACHE | BDRV_O_CACHE_WB;
         } else if (!strcmp(buf, "writeback")) {
             bdrv_flags |= BDRV_O_CACHE_WB;
         } else if (!strcmp(buf, "unsafe")) {

Index: qemu/qemu-io.c
===================================================================
@@ -1655,7 +1655,7 @@ open_f(int argc, char **argv)
 			flags |= BDRV_O_SNAPSHOT;
 			break;
 		case 'n':
-			flags |= BDRV_O_NOCACHE;
+			flags |= BDRV_O_NOCACHE | BDRV_O_CACHE_WB;
 			break;
 		case 'r':
 			readonly = 1;

@@ -1751,7 +1751,7 @@ int main(int argc, char **argv)
 			flags |= BDRV_O_SNAPSHOT;
 			break;
 		case 'n':
-			flags |= BDRV_O_NOCACHE;
+			flags |= BDRV_O_NOCACHE | BDRV_O_CACHE_WB;
 			break;
 		case 'c':
 			add_user_command(optarg);

Index: qemu/qemu-nbd.c
===================================================================
@@ -238,7 +238,7 @@ int main(int argc, char **argv)
             flags |= BDRV_O_SNAPSHOT;
             break;
         case 'n':
-            flags |= BDRV_O_NOCACHE;
+            flags |= BDRV_O_NOCACHE | BDRV_O_CACHE_WB;
             break;
         case 'b':
             bindto = optarg;

Index: qemu/block/qcow2.c
===================================================================
@@ -229,7 +229,7 @@ static int qcow2_open(BlockDriverState *
     }
 
     /* alloc L2 table/refcount block cache */
-    writethrough = ((flags & BDRV_O_CACHE_MASK) == 0);
+    writethrough = ((flags & BDRV_O_CACHE_WB) == 0);
     s->l2_table_cache = qcow2_cache_create(bs, L2_CACHE_SIZE, writethrough);
     s->refcount_block_cache = qcow2_cache_create(bs, REFCOUNT_CACHE_SIZE,
         writethrough);