block: Move enable_write_cache to BB level

Whether a write cache is used or not is a decision that concerns the
user (e.g. the guest device) rather than the backend. It was already
logically part of the BB level as bdrv_move_feature_fields() always kept
it on top of the BDS tree; with this patch, the core of it (the actual
flag and the additional flushes) is also implemented there.

Direct callers of bdrv_open() must pass BDRV_O_CACHE_WB now if bs
doesn't have a BlockBackend attached.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

3 files changed, 29 insertions, 17 deletions

diff --git a/block/block-backend.c b/block/block-backend.c
index 048f48e935..a263636a58 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -47,6 +47,8 @@ struct BlockBackend {
      * can be used to restore those options in the new BDS on insert) */
     BlockBackendRootState root_state;
 
+    bool enable_write_cache;
+
     /* I/O stats (display with "info blockstats"). */
     BlockAcctStats stats;
 
@@ -699,11 +701,17 @@ static int coroutine_fn blk_co_pwritev(BlockBackend *blk, int64_t offset,
                                       unsigned int bytes, QEMUIOVector *qiov,
                                       BdrvRequestFlags flags)
 {
-    int ret = blk_check_byte_request(blk, offset, bytes);
+    int ret;
+
+    ret = blk_check_byte_request(blk, offset, bytes);
     if (ret < 0) {
         return ret;
     }
 
+    if (!blk->enable_write_cache) {
+        flags |= BDRV_REQ_FUA;
+    }
+
     return bdrv_co_do_pwritev(blk_bs(blk), offset, bytes, qiov, flags);
 }
 
@@ -1210,26 +1218,19 @@ int blk_is_sg(BlockBackend *blk)
 
 int blk_enable_write_cache(BlockBackend *blk)
 {
-    BlockDriverState *bs = blk_bs(blk);
-
-    if (bs) {
-        return bdrv_enable_write_cache(bs);
-    } else {
-        return !!(blk->root_state.open_flags & BDRV_O_CACHE_WB);
-    }
+    return blk->enable_write_cache;
 }
 
 void blk_set_enable_write_cache(BlockBackend *blk, bool wce)
 {
-    BlockDriverState *bs = blk_bs(blk);
+    blk->enable_write_cache = wce;
 
-    if (bs) {
-        bdrv_set_enable_write_cache(bs, wce);
-    } else {
+    /* TODO Remove this when BDRV_O_CACHE_WB isn't used any more */
+    if (blk->root) {
         if (wce) {
-            blk->root_state.open_flags |= BDRV_O_CACHE_WB;
+            blk->root->bs->open_flags |= BDRV_O_CACHE_WB;
         } else {
-            blk->root_state.open_flags &= ~BDRV_O_CACHE_WB;
+            blk->root->bs->open_flags &= ~BDRV_O_CACHE_WB;
         }
     }
 }
@@ -1492,11 +1493,22 @@ int blk_discard(BlockBackend *blk, int64_t sector_num, int nb_sectors)
 int blk_save_vmstate(BlockBackend *blk, const uint8_t *buf,
                      int64_t pos, int size)
 {
+    int ret;
+
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
-    return bdrv_save_vmstate(blk_bs(blk), buf, pos, size);
+    ret = bdrv_save_vmstate(blk_bs(blk), buf, pos, size);
+    if (ret < 0) {
+        return ret;
+    }
+
+    if (ret == size && !blk->enable_write_cache) {
+        ret = bdrv_flush(blk_bs(blk));
+    }
+
+    return ret < 0 ? ret : size;
 }
 
 int blk_load_vmstate(BlockBackend *blk, uint8_t *buf, int64_t pos, int size)
diff --git a/block/io.c b/block/io.c
index b58843594e..939e4f1559 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1160,7 +1160,7 @@ static int coroutine_fn bdrv_aligned_pwritev(BlockDriverState *bs,
     }
     bdrv_debug_event(bs, BLKDBG_PWRITEV_DONE);
 
-    if (ret == 0 && !bs->enable_write_cache) {
+    if (ret == 0 && (flags & BDRV_REQ_FUA)) {
         ret = bdrv_co_flush(bs);
     }
 
diff --git a/block/iscsi.c b/block/iscsi.c
index 128ea79c13..3b5453687d 100644
--- a/block/iscsi.c
+++ b/block/iscsi.c
@@ -476,7 +476,7 @@ static int coroutine_fn iscsi_co_writev(BlockDriverState *bs,
     num_sectors = sector_qemu2lun(nb_sectors, iscsilun);
     iscsi_co_init_iscsitask(iscsilun, &iTask);
 retry:
-    fua = iscsilun->dpofua && !bs->enable_write_cache;
+    fua = iscsilun->dpofua && !bdrv_enable_write_cache(bs);
     iTask.force_next_flush = !fua;
     if (iscsilun->use_16_for_rw) {
         iTask.task = iscsi_write16_task(iscsilun->iscsi, iscsilun->lun, lba,