qcow2: always operate caches in writeback mode

Writethrough does not need special-casing anymore in the qcow2 caches.
The block layer adds flushes after every guest-initiated data write,
and these will also flush the qcow2 caches to the OS.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-cache.c

@@ -40,11 +40,9 @@ struct Qcow2Cache {
     struct Qcow2Cache*      depends;
     int                     size;
     bool                    depends_on_flush;
-    bool                    writethrough;
 };
 
-Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables,
+Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables)
-    bool writethrough)
 {
     BDRVQcowState *s = bs->opaque;
     Qcow2Cache *c;
@@ -53,7 +51,6 @@ Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables,
     c = g_malloc0(sizeof(*c));
     c->size = num_tables;
     c->entries = g_malloc0(sizeof(*c->entries) * num_tables);
-    c->writethrough = writethrough;
 
     for (i = 0; i < c->size; i++) {
         c->entries[i].table = qemu_blockalign(bs, s->cluster_size);
@@ -307,12 +304,7 @@ found:
     *table = NULL;
 
     assert(c->entries[i].ref >= 0);
-
-    if (c->writethrough) {
-        return qcow2_cache_entry_flush(bs, c, i);
-    } else {
     return 0;
-    }
 }
 
 void qcow2_cache_entry_mark_dirty(Qcow2Cache *c, void *table)
@@ -329,16 +321,3 @@ void qcow2_cache_entry_mark_dirty(Qcow2Cache *c, void *table)
 found:
     c->entries[i].dirty = true;
 }
-
-bool qcow2_cache_set_writethrough(BlockDriverState *bs, Qcow2Cache *c,
-    bool enable)
-{
-    bool old = c->writethrough;
-
-    if (!old && enable) {
-        qcow2_cache_flush(bs, c);
-    }
-
-    c->writethrough = enable;
-    return old;
-}

block/qcow2-refcount.c

@@ -726,13 +726,6 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
     int64_t old_offset, old_l2_offset;
     int i, j, l1_modified = 0, nb_csectors, refcount;
     int ret;
-    bool old_l2_writethrough, old_refcount_writethrough;
-
-    /* Switch caches to writeback mode during update */
-    old_l2_writethrough =
-        qcow2_cache_set_writethrough(bs, s->l2_table_cache, false);
-    old_refcount_writethrough =
-        qcow2_cache_set_writethrough(bs, s->refcount_block_cache, false);
 
     l2_table = NULL;
     l1_table = NULL;
@@ -856,11 +849,6 @@ fail:
         qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
     }
 
-    /* Enable writethrough cache mode again */
-    qcow2_cache_set_writethrough(bs, s->l2_table_cache, old_l2_writethrough);
-    qcow2_cache_set_writethrough(bs, s->refcount_block_cache,
-        old_refcount_writethrough);
-
     /* Update L1 only if it isn't deleted anyway (addend = -1) */
     if (addend >= 0 && l1_modified) {
         for(i = 0; i < l1_size; i++)

block/qcow2.c

@@ -220,7 +220,6 @@ static int qcow2_open(BlockDriverState *bs, int flags)
     int len, i, ret = 0;
     QCowHeader header;
     uint64_t ext_end;
-    bool writethrough;
 
     ret = bdrv_pread(bs->file, 0, &header, sizeof(header));
     if (ret < 0) {
@@ -367,10 +366,8 @@ static int qcow2_open(BlockDriverState *bs, int flags)
     }
 
     /* alloc L2 table/refcount block cache */
-    writethrough = ((flags & BDRV_O_CACHE_WB) == 0);
+    s->l2_table_cache = qcow2_cache_create(bs, L2_CACHE_SIZE);
-    s->l2_table_cache = qcow2_cache_create(bs, L2_CACHE_SIZE, writethrough);
+    s->refcount_block_cache = qcow2_cache_create(bs, REFCOUNT_CACHE_SIZE);
-    s->refcount_block_cache = qcow2_cache_create(bs, REFCOUNT_CACHE_SIZE,
-        writethrough);
 
     s->cluster_cache = g_malloc(s->cluster_size);
     /* one more sector for decompressed data alignment */

block/qcow2.h

@@ -297,11 +297,8 @@ void qcow2_free_snapshots(BlockDriverState *bs);
 int qcow2_read_snapshots(BlockDriverState *bs);
 
 /* qcow2-cache.c functions */
-Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables,
+Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables);
-    bool writethrough);
 int qcow2_cache_destroy(BlockDriverState* bs, Qcow2Cache *c);
-bool qcow2_cache_set_writethrough(BlockDriverState *bs, Qcow2Cache *c,
-    bool enable);
 
 void qcow2_cache_entry_mark_dirty(Qcow2Cache *c, void *table);
 int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c);