[PATCH 11/11] block: introduce fleecing block driver

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Introduce a new driver, that works in pair with copy-before-write to
improve fleecing.

Without fleecing driver, fleecing scheme may look as follows:

[guest]
  |
  |root
  v
[copy-before-write] -----> [temp.qcow2] <--- [nbd export]
  |                 target  |
  |file                     |backing
  v                         |
[active disk] <-------------+

With fleecing driver, new scheme is:

           bdrv_ref()
[guest]<~~~~~~~~~~~~~~~~~~~[fleecing] <--- [nbd export]
  |                         |    |
  |root      +--source------+    |file
  v          v                   v
[copy-before-write] -----> [temp.img]
  |                 target
  |file
  v
[active disk]

Benefits of new scheme:

1. Access control: if remote client try to read data that not covered
   by original dirty bitmap used on copy-before-write open, client gets
   -EACCES.

2. Discard support: if remote client do DISCARD, this additionally to
   discarding data in temp.img informs block-copy process to not copy
   these clusters. Next read from discarded area will return -EACCES.

3. Synchronisation between client reads and block-copy write is more
   efficient: it doesn't block intersecting block-copy write during
   client read (hmm, we still needlesly block it, as block-copy
   always serialize writes, it's a TODO to stop doing so).

4. We don't rely on backing feature: active disk should not be backing
   of temp image, so we avoid some permission-related difficulties
   (cleaning them up in copy-before-write filter is a TODO) and temp
   image now is not required to support backing, it may be simple raw
   image.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/fleecing.c  | 183 ++++++++++++++++++++++++++++++++++++++++++++++
 block/meson.build |   1 +
 2 files changed, 184 insertions(+)
 create mode 100644 block/fleecing.c

diff --git a/block/fleecing.c b/block/fleecing.c
new file mode 100644
index 0000000000..7d213da1ca
--- /dev/null
+++ b/block/fleecing.c
@@ -0,0 +1,183 @@
+/*
+ * copy-before-write filter driver
+ *
+ * The driver performs Copy-Before-Write (CBW) operation: it is injected above
+ * some node, and before each write it copies _old_ data to the target node.
+ *
+ * Copyright (c) 2018-2021 Virtuozzo International GmbH.
+ *
+ * Author:
+ *  Sementsov-Ogievskiy Vladimir <vsementsov@virtuozzo.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "qemu/osdep.h"
+
+#include "sysemu/block-backend.h"
+#include "qemu/cutils.h"
+#include "qapi/error.h"
+#include "block/block_int.h"
+#include "block/qdict.h"
+#include "block/block-copy.h"
+
+#include "block/copy-before-write.h"
+
+typedef struct BDRVFleecingState {
+    BlockDriverState *cbw;
+    BdrvChild *source;
+} BDRVFleecingState;
+
+static coroutine_fn int fleecing_co_preadv_part(
+        BlockDriverState *bs, uint64_t offset, uint64_t bytes,
+        QEMUIOVector *qiov, size_t qiov_offset, int flags)
+{
+    BDRVFleecingState *s = bs->opaque;
+    const BlockReq *req;
+    int ret;
+
+    /* TODO: upgrade to async loop using AioTask */
+    while (bytes) {
+        int64_t cur_bytes;
+
+        ret = cbw_snapshot_read_lock(s->cbw, offset, bytes, &req, &cur_bytes);
+        if (ret < 0) {
+            return ret;
+        }
+
+        if (req) {
+            ret = bdrv_co_preadv_part(s->source, offset, cur_bytes,
+                                      qiov, qiov_offset, flags);
+            cbw_snapshot_read_unlock(s->cbw, req);
+        } else {
+            ret = bdrv_co_preadv_part(bs->file, offset, cur_bytes,
+                                      qiov, qiov_offset, flags);
+        }
+        if (ret < 0) {
+            return ret;
+        }
+
+        bytes -= cur_bytes;
+        offset += cur_bytes;
+        qiov_offset += cur_bytes;
+    }
+
+    return 0;
+}
+
+static int coroutine_fn fleecing_co_pdiscard(BlockDriverState *bs,
+                                             int64_t offset, int bytes)
+{
+    BDRVFleecingState *s = bs->opaque;
+
+    cbw_snapshot_discard(s->cbw, offset, bytes);
+
+    bdrv_co_pdiscard(bs->file, offset, bytes);
+
+    /*
+     * Ignore bdrv_co_pdiscard() result: cbw_snapshot_discard() succeeded, that
+     * means that next read from this area will fail with -EACCES. More correct
+     * to report success now.
+     */
+    return 0;
+}
+
+static int coroutine_fn fleecing_co_pwrite_zeroes(BlockDriverState *bs,
+        int64_t offset, int bytes, BdrvRequestFlags flags)
+{
+    return -EACCES;
+}
+
+static coroutine_fn int fleecing_co_pwritev(BlockDriverState *bs,
+                                       uint64_t offset,
+                                       uint64_t bytes,
+                                       QEMUIOVector *qiov, int flags)
+{
+    return -EACCES;
+}
+
+
+static void fleecing_refresh_filename(BlockDriverState *bs)
+{
+    pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
+            bs->file->bs->filename);
+}
+
+static int fleecing_open(BlockDriverState *bs, QDict *options, int flags,
+                         Error **errp)
+{
+    BDRVFleecingState *s = bs->opaque;
+    const char *cbw_node_name = qdict_get_str(options,
+                                              "copy-before-write-node");
+    BlockDriverState *cbw;
+
+    cbw = bdrv_find_node(cbw_node_name);
+    if (!cbw) {
+        error_setg(errp, "Node '%s' not found", cbw_node_name);
+        return -EINVAL;
+    }
+
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
+                               BDRV_CHILD_DATA | BDRV_CHILD_PRIMARY,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
+    s->source = bdrv_open_child(NULL, options, "source", bs, &child_of_bds,
+                               BDRV_CHILD_DATA, false, errp);
+    if (!s->source) {
+        return -EINVAL;
+    }
+
+    bs->total_sectors = bs->file->bs->total_sectors;
+
+    s->cbw = cbw;
+    bdrv_ref(cbw);
+
+    return 0;
+}
+
+static void fleecing_close(BlockDriverState *bs)
+{
+    BDRVFleecingState *s = bs->opaque;
+
+    bdrv_unref(s->cbw);
+}
+
+BlockDriver bdrv_fleecing_filter = {
+    .format_name = "fleecing",
+    .instance_size = sizeof(BDRVFleecingState),
+
+    .bdrv_open                  = fleecing_open,
+    .bdrv_close                 = fleecing_close,
+
+    .bdrv_co_preadv_part        = fleecing_co_preadv_part,
+    .bdrv_co_pwritev            = fleecing_co_pwritev,
+    .bdrv_co_pwrite_zeroes      = fleecing_co_pwrite_zeroes,
+    .bdrv_co_pdiscard           = fleecing_co_pdiscard,
+
+    .bdrv_refresh_filename      = fleecing_refresh_filename,
+
+    .bdrv_child_perm            = bdrv_default_perms,
+
+    .is_filter = true,
+};
+
+static void fleecing_init(void)
+{
+    bdrv_register(&bdrv_fleecing_filter);
+}
+
+block_init(fleecing_init);
diff --git a/block/meson.build b/block/meson.build
index c630e9374a..caa52f49c2 100644
--- a/block/meson.build
+++ b/block/meson.build
@@ -18,6 +18,7 @@ block_ss.add(files(
   'crypto.c',
   'dirty-bitmap.c',
   'filter-compress.c',
+  'fleecing.c',
   'io.c',
   'mirror.c',
   'nbd.c',
-- 
2.29.2