block: Introduce "null" drivers

This is an analogue to Linux null_blk. It can be used for testing or
benchmarking block device emulation and general block layer
functionalities such as coroutines and throttling, where disk IO is not
necessary or wanted.

Use null-aio:// for AIO version, and null-co:// for coroutine version.

[Resolved conflict with Fam's async bdrv_aio_cancel() series:
1. Drop .bdrv_aio_cancel() since it is now done by block.c
2. Rename qemu_aio_release() to qemu_aio_unref()
--Stefan]

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1410415798-20673-2-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/Makefile.objs

@@ -9,6 +9,7 @@ block-obj-y += snapshot.o qapi.o
 block-obj-$(CONFIG_WIN32) += raw-win32.o win32-aio.o
 block-obj-$(CONFIG_POSIX) += raw-posix.o
 block-obj-$(CONFIG_LINUX_AIO) += linux-aio.o
+block-obj-y += null.o
 
 block-obj-y += nbd.o nbd-client.o sheepdog.o
 block-obj-$(CONFIG_LIBISCSI) += iscsi.o

block/null.c

@@ -0,0 +1,168 @@
+/*
+ * Null block driver
+ *
+ * Authors:
+ *  Fam Zheng <famz@redhat.com>
+ *
+ * Copyright (C) 2014 Red Hat, Inc.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "block/block_int.h"
+
+typedef struct {
+    int64_t length;
+} BDRVNullState;
+
+static QemuOptsList runtime_opts = {
+    .name = "null",
+    .head = QTAILQ_HEAD_INITIALIZER(runtime_opts.head),
+    .desc = {
+        {
+            .name = "filename",
+            .type = QEMU_OPT_STRING,
+            .help = "",
+        },
+        {
+            .name = BLOCK_OPT_SIZE,
+            .type = QEMU_OPT_SIZE,
+            .help = "size of the null block",
+        },
+        { /* end of list */ }
+    },
+};
+
+static int null_file_open(BlockDriverState *bs, QDict *options, int flags,
+                          Error **errp)
+{
+    QemuOpts *opts;
+    BDRVNullState *s = bs->opaque;
+
+    opts = qemu_opts_create(&runtime_opts, NULL, 0, &error_abort);
+    qemu_opts_absorb_qdict(opts, options, &error_abort);
+    s->length =
+        qemu_opt_get_size(opts, BLOCK_OPT_SIZE, 1 << 30);
+    qemu_opts_del(opts);
+    return 0;
+}
+
+static void null_close(BlockDriverState *bs)
+{
+}
+
+static int64_t null_getlength(BlockDriverState *bs)
+{
+    BDRVNullState *s = bs->opaque;
+    return s->length;
+}
+
+static coroutine_fn int null_co_readv(BlockDriverState *bs,
+                                      int64_t sector_num, int nb_sectors,
+                                      QEMUIOVector *qiov)
+{
+    return 0;
+}
+
+static coroutine_fn int null_co_writev(BlockDriverState *bs,
+                                       int64_t sector_num, int nb_sectors,
+                                       QEMUIOVector *qiov)
+{
+    return 0;
+}
+
+static coroutine_fn int null_co_flush(BlockDriverState *bs)
+{
+    return 0;
+}
+
+typedef struct {
+    BlockDriverAIOCB common;
+    QEMUBH *bh;
+} NullAIOCB;
+
+static const AIOCBInfo null_aiocb_info = {
+    .aiocb_size = sizeof(NullAIOCB),
+};
+
+static void null_bh_cb(void *opaque)
+{
+    NullAIOCB *acb = opaque;
+    acb->common.cb(acb->common.opaque, 0);
+    qemu_bh_delete(acb->bh);
+    qemu_aio_unref(acb);
+}
+
+static inline BlockDriverAIOCB *null_aio_common(BlockDriverState *bs,
+                                                BlockDriverCompletionFunc *cb,
+                                                void *opaque)
+{
+    NullAIOCB *acb;
+
+    acb = qemu_aio_get(&null_aiocb_info, bs, cb, opaque);
+    acb->bh = aio_bh_new(bdrv_get_aio_context(bs), null_bh_cb, acb);
+    qemu_bh_schedule(acb->bh);
+    return &acb->common;
+}
+
+static BlockDriverAIOCB *null_aio_readv(BlockDriverState *bs,
+                                        int64_t sector_num, QEMUIOVector *qiov,
+                                        int nb_sectors,
+                                        BlockDriverCompletionFunc *cb,
+                                        void *opaque)
+{
+    return null_aio_common(bs, cb, opaque);
+}
+
+static BlockDriverAIOCB *null_aio_writev(BlockDriverState *bs,
+                                         int64_t sector_num, QEMUIOVector *qiov,
+                                         int nb_sectors,
+                                         BlockDriverCompletionFunc *cb,
+                                         void *opaque)
+{
+    return null_aio_common(bs, cb, opaque);
+}
+
+static BlockDriverAIOCB *null_aio_flush(BlockDriverState *bs,
+                                        BlockDriverCompletionFunc *cb,
+                                        void *opaque)
+{
+    return null_aio_common(bs, cb, opaque);
+}
+
+static BlockDriver bdrv_null_co = {
+    .format_name            = "null-co",
+    .protocol_name          = "null-co",
+    .instance_size          = sizeof(BDRVNullState),
+
+    .bdrv_file_open         = null_file_open,
+    .bdrv_close             = null_close,
+    .bdrv_getlength         = null_getlength,
+
+    .bdrv_co_readv          = null_co_readv,
+    .bdrv_co_writev         = null_co_writev,
+    .bdrv_co_flush_to_disk  = null_co_flush,
+};
+
+static BlockDriver bdrv_null_aio = {
+    .format_name            = "null-aio",
+    .protocol_name          = "null-aio",
+    .instance_size          = sizeof(BDRVNullState),
+
+    .bdrv_file_open         = null_file_open,
+    .bdrv_close             = null_close,
+    .bdrv_getlength         = null_getlength,
+
+    .bdrv_aio_readv         = null_aio_readv,
+    .bdrv_aio_writev        = null_aio_writev,
+    .bdrv_aio_flush         = null_aio_flush,
+};
+
+static void bdrv_null_init(void)
+{
+    bdrv_register(&bdrv_null_co);
+    bdrv_register(&bdrv_null_aio);
+}
+
+block_init(bdrv_null_init);

qapi/block-core.json

@@ -1151,7 +1151,8 @@
   'data': [ 'archipelago', 'file', 'host_device', 'host_cdrom', 'host_floppy',
             'http', 'https', 'ftp', 'ftps', 'tftp', 'vvfat', 'blkdebug',
             'blkverify', 'bochs', 'cloop', 'cow', 'dmg', 'parallels', 'qcow',
-            'qcow2', 'qed', 'raw', 'vdi', 'vhdx', 'vmdk', 'vpc', 'quorum' ] }
+            'qcow2', 'qed', 'raw', 'vdi', 'vhdx', 'vmdk', 'vpc', 'quorum',
+            'null-aio', 'null-co' ] }
 
 ##
 # @BlockdevOptionsBase
@@ -1203,6 +1204,18 @@
 { 'type': 'BlockdevOptionsFile',
   'data': { 'filename': 'str' } }
 
+##
+# @BlockdevOptionsNull
+#
+# Driver specific block device options for the null backend.
+#
+# @size:    #optional size of the device in bytes.
+#
+# Since: 2.2
+##
+{ 'type': 'BlockdevOptionsNull',
+  'data': { '*size': 'int' } }
+
 ##
 # @BlockdevOptionsVVFAT
 #
@@ -1503,7 +1516,9 @@
       'vhdx':       'BlockdevOptionsGenericFormat',
       'vmdk':       'BlockdevOptionsGenericCOWFormat',
       'vpc':        'BlockdevOptionsGenericFormat',
-      'quorum':     'BlockdevOptionsQuorum'
+      'quorum':     'BlockdevOptionsQuorum',
+      'null-aio':   'BlockdevOptionsNull',
+      'null-co':    'BlockdevOptionsNull'
   } }
 
 ##