[Qemu-devel] [PATCH v3 1/8] block: add bdrv_add_before_write_cb()

[Stefan Hajnoczi]

The bdrv_add_before_write_cb() function installs a callback that is
invoked before a write request is processed.  This will be used to
implement copy-on-write point-in-time snapshots where we need to copy
out old data before overwriting it.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 block.c                   | 37 +++++++++++++++++++++++++++++++++++++
 include/block/block_int.h | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+)

diff --git a/block.c b/block.c
index 3f87489..0fd7167 100644
--- a/block.c
+++ b/block.c
@@ -308,6 +308,7 @@ BlockDriverState *bdrv_new(const char *device_name)
     }
     bdrv_iostatus_disable(bs);
     notifier_list_init(&bs->close_notifiers);
+    QTAILQ_INIT(&bs->before_write_cbs);
 
     return bs;
 }
@@ -1383,6 +1384,8 @@ void bdrv_close(BlockDriverState *bs)
         bs->growable = 0;
         QDECREF(bs->options);
         bs->options = NULL;
+        assert(QTAILQ_EMPTY(&bs->before_write_cbs));
+        QTAILQ_INIT(&bs->before_write_cbs);
 
         if (bs->file != NULL) {
             bdrv_delete(bs->file);
@@ -2587,6 +2590,22 @@ static int coroutine_fn 
bdrv_co_do_write_zeroes(BlockDriverState *bs,
     return ret;
 }
 
+struct BDRVBeforeWrite {
+    BDRVBeforeWriteFunc *cb;
+    QTAILQ_ENTRY(BDRVBeforeWrite) list;
+};
+
+static void invoke_before_write_cb(BlockDriverState *bs, int64_t sector_num,
+                                   int nb_sectors, QEMUIOVector *qiov)
+{
+    BDRVBeforeWrite *before_write;
+    BDRVBeforeWrite *tmp;
+    QTAILQ_FOREACH_SAFE(before_write, &bs->before_write_cbs, list, tmp) {
+        before_write->cb(bs, sector_num, nb_sectors, qiov);
+    }
+}
+
+
 /*
  * Handle a write request in coroutine context
  */
@@ -2619,6 +2638,8 @@ static int coroutine_fn 
bdrv_co_do_writev(BlockDriverState *bs,
 
     tracked_request_begin(&req, bs, sector_num, nb_sectors, true);
 
+    invoke_before_write_cb(bs, sector_num, nb_sectors, qiov);
+
     if (flags & BDRV_REQ_ZERO_WRITE) {
         ret = bdrv_co_do_write_zeroes(bs, sector_num, nb_sectors);
     } else {
@@ -4883,3 +4904,19 @@ AioContext *bdrv_get_aio_context(BlockDriverState *bs)
     /* Currently BlockDriverState always uses the main loop AioContext */
     return qemu_get_aio_context();
 }
+
+BDRVBeforeWrite *bdrv_add_before_write_cb(BlockDriverState *bs,
+                                          BDRVBeforeWriteFunc *cb)
+{
+    BDRVBeforeWrite *elem = g_slice_new(BDRVBeforeWrite);
+    elem->cb = cb;
+    QTAILQ_INSERT_TAIL(&bs->before_write_cbs, elem, list);
+    return elem;
+}
+
+void bdrv_remove_before_write_cb(BlockDriverState *bs,
+                                 BDRVBeforeWrite *before_write)
+{
+    QTAILQ_REMOVE(&bs->before_write_cbs, before_write, list);
+    g_slice_free(BDRVBeforeWrite, before_write);
+}
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 6078dd3..e2299df 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -211,6 +211,16 @@ struct BlockDriver {
     QLIST_ENTRY(BlockDriver) list;
 };
 
+/**
+ * BDRVBeforeWriteFunc:
+ *
+ * See #bdrv_add_before_write_cb().
+ */
+typedef void coroutine_fn BDRVBeforeWriteFunc(BlockDriverState *bs,
+        int64_t sector_num, int nb_sectors, QEMUIOVector *qiov);
+
+typedef struct BDRVBeforeWrite BDRVBeforeWrite;
+
 /*
  * Note: the function bdrv_append() copies and swaps contents of
  * BlockDriverStates, so if you add new fields to this struct, please
@@ -289,6 +299,9 @@ struct BlockDriverState {
     /* long-running background operation */
     BlockJob *job;
 
+    /* Callback before write request is processed */
+    QTAILQ_HEAD(, BDRVBeforeWrite) before_write_cbs;
+
     QDict *options;
 };
 
@@ -298,6 +311,25 @@ void bdrv_set_io_limits(BlockDriverState *bs,
                         BlockIOLimit *io_limits);
 
 /**
+ * bdrv_add_before_write_cb:
+ *
+ * Register a callback that is invoked before write requests are processed but
+ * after any throttling or waiting for overlapping requests.
+ *
+ * Returns: a #BDRVBeforeWrite to use with bdrv_remove_before_write_cb()
+ */
+BDRVBeforeWrite *bdrv_add_before_write_cb(BlockDriverState *bs,
+                                          BDRVBeforeWriteFunc *cb);
+
+/**
+ * bdrv_remove_before_write_cb:
+ *
+ * Unregister a before write callback.
+ */
+void bdrv_remove_before_write_cb(BlockDriverState *bs,
+                                 BDRVBeforeWrite *before_write);
+
+/**
  * bdrv_get_aio_context:
  *
  * Returns: the currently bound #AioContext
-- 
1.8.1.4